Gale Warning
Issued at: 5:00 PM TODAY , 19 February 2017
Gale Warning # 3 (Final)
For: strong to gale force winds associated with the northeast monsoon.

Gale Warning in PDF file



Weather Advisory
As of today, there is no Weather Advisory issued.


Dams Water Level Update
As of 6 AM, 22 February  2017

General Flood Advisories - Regional
Issued 22 February 2017


Daily Basin Hydrological Forecast
Issued 22 February 2017




Monthly Climate Assessment and Outlook

La Niña-like conditions continue
Issued: 06 February 2017
updates shall be issued as appropriate.

Monthly Rainfall Forecast
RAINFALL FORECAST  (February - July 2017) 
UPDATED: 18 January 2017 


Regional Rainfall Forecast
Issued: 18 January 2017
Valid for: February - July 2017
Farm Weather Forecast and Advisories
ISSUED              : 8AM, WEDNESDAY, FEBRUARY 22, 2017
VALID UNTIL      :  8AM, THURSDAY, FEBRUARY 23, 2017
FWFA:  N0. 17-053


Ten-Day Regional Agri-Weather Information

DEKAD NO. 6  FEBRUARY 21- 28, 2017

PHILIPPINE AGRI-WEATHER FORECAST

The weather systems that will affect the whole country are northeast monsoon, easterlies, low pressure area (LPA) and tail-end of a cold front.

The whole country will have partly cloudy to cloudy skies with isolated rains or thunderstorms during the early days of the forecast period. After this, Mindanao and the regions of MIMAROPA, Eastern Visayas and Central Visayas will have light to moderate rains and thunderstorms. The regions of Cagayan Valley, Ilocos, CAR, Central Luzon and Bicol will experience cloudiness which may bring rains or thunderstorms at the midst of the forecast period up to the latter part of the dekad. The rest of the country will have partly cloudy to cloudy skies with isolated rainshowers or thunderstorms.

Light to moderate winds will blow from east to southeast over Luzon during the early days of the dekadal period. After this, gradual shift to northeasterly direction of moderate to strong winds will prevail over Luzon. The rest of the country will have winds coming from east to northeast.

During the early days of the forecast period, the seaboards of the whole country will be slight to moderate. The coastal waters of Luzon will be moderate to rough at the midst of the forecast period. This will take place up to the latter part of the dekad. The rest of the country will have slight to moderate seas.

The eastern and central equatorial Pacific is in neutral condition.


Seasonal Climate Outlook
Issued:  20 January 2017
FOR January - June 2017
PDF 




Astronomical Diary
Issued 01 February 2017
National Astronomy Week (NAW), Famous Equilateral triangle in the sky and Annular Solar Eclipse (not visible in the Philippines)...




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Meteorology is the science that deals with the earth's atmosphere and
its phenomena; specifically that of weather and climate and its effects
on the earth's surface, oceans and life in general.
The materials in this web page were compiled and simplified to help our
readers gain a better understanding of the information disseminated and
services offered by PAGASA for people from all walks of life. On a broader
scope, this web page aims to provide its readers insight on the dynamics
of the weather which changes on a daily basis and of the climate which
changes over a greater period of time.


A weather forecast is simply a scientific estimate of future weather condition.  Weather condition is the state of the atmosphere at a given time expressed in terms of the most significant weather variables.  The significant weather variables being forecast differ from place to place.   In the Philippines, the weather parameters with significant variation and therefore of interest to the users of the forecast are cloudiness, rainfall and wind.


In forecasting the weather, a Meteorologist must at least know something about the existing weather condition over a large area before he can make a reliable forecast.  The accuracy of his forecast depends largely upon his knowledge of the prevailing weather conditions over a very wide area.  The forecast decision is based on various forecasting tools.  The basic tool of a weather forecaster is the WEATHER MAP.  The weather map depicts the distribution patterns of atmospheric pressure, wind, temperature and humidity at the different levels of the atmosphere.  There are two types of the basic weather map namely, the surface map and the upper-air maps.  There are five standard levels of the upper-air maps that are constructed twice daily at twelve-hourly interval.  The surface maps are made four times daily at six-hourly intervals.  On the surface maps, the distribution patterns of rain or other forms of precipitation and cloudiness can also be delineated.

1st Step: Observation
       Observation of different weather elements are made simultaneously as follows:

  • Surface Observation Surface observations are made at least every three hours over land and sea.  Land-based weather stations around the world and automatic stations observe the atmospheric pressure, wind direction and speed, temperature of the air, humidity, clouds, precipitation and visibility using standard weather instruments such as the barometer, wind vane, anemometer, thermometer, psychrometer or hygrometer and raingauge.  In addition to these, coastal weather stations, weather ships and ocean data buoy observe the state of the sea by observing the height and period of wave.

  • Weather balloonUpper air stations around the world also make observations at least every twelve hours.  The pressure, temperature, dew point temperature, wind direction and speed are observed at selected levels in the atmosphere using radiosondes which record these data by tracking helium-filled balloons attached to transmitters.  Another apparatus, the theodolite, is used in observing wind direction and speed also at selected levels.  In addition to these, commercial air planes observe the weather along their routes at specified times.

  • Satellite Room Meteorological satellites, geostationary and polar orbiting, take pictures of the cloud imagery of the atmosphere.  These satellites take picture of the earth's cloud formations every hour and continuously, respectively.

  • Weather radars are also used to observe the cloud coverage within the range of the radar.

  • A vast array of weather data are fed to the computer which analyzes them as programmed and makes a time integration of physical equations.  This is called numerical weather prediction.

2nd Step: Collection And Transmission Of Weather Data

Receiving of Weather Observation        Weather observations which are condensed into coded figures, symbols and numerals are transmitted via radiophone, teletype, facsimile machine or telephone to designated collection centers for further transmission to the central forecasting station at WFFC.  Weather satellite pictures are transmitted to ground receiving stations while radar observations are transmitted to forecasting centers through a local communication system.

3rd Step: Plotting Of Weather Data

Plotting weather map        Upon receipt of the coded messages, they are decoded and each set of observations is plotted in symbols or numbers on weather charts over the respective areas or regions.  Observations made over land and sea are plotted on the surface or mean sea level charts which are prepared four times a day.  Radiosonde, theodolite, aircraft and satellite wind observations are plotted on upper level charts which are prepared twice daily.

4th Step: Analysis Of Weather Maps, Satellite And Radar Imageries And Other Data
Analysing weather map Current weather maps are analyzed as follows:
  • SURFACE (MSL) CHART: The data plotted on this weather map are analyzed isobarically.  This means the same atmospheric pressure at different places are inter-connected with a line taking into consideration the direction of the wind.  Through this analysis, weather systems or the so-called centers of action such as high and low pressure areas, tropical cyclones, cold and warm fronts, intertropical convergence zone, can be located and delineated.

  • UPPER AIR CHARTS: The data plotted on this weather map are analyzed using streamline analysis.  Lines are drawn to illustrate the flow of the wind.  With this kind of analysis, anticyclones or high pressure areas and cyclones or low pressure areas can be delineated.

  • NUMERICAL WEATHER PREDICTION MODEL OUTPUT: The computer-plotted weather maps are analyzed manually so that weather systems like cyclones and anticyclones, troughs, etc. are located.

  • MONITOR WEATHER CHARTS: Plotted data on the cross-section, rainfall and 24-hour pressure change charts are analyzed to determine the movement of wind waves, rainfall distribution and the behavior of the atmospheric pressure.

Compare the current weather maps with the previous 24 - 72 hour weather maps level by level to determine the development and movement of weather systems that may affect the forecast area.

Examine the latest weather satellite picture, noting the cloud formations in relation to the weather systems on the current weather maps.

Compare the latest weather satellite picture with the previous satellite pictures (up to 48 hours) noting the development and movement of weather systems that may affect the country.

Examine the latest computer output of the numerical weather prediction model noting the 24-hour, 48-hour and 72-hour objective forecast of the weather systems that may affect the forecast area.

Analyze the latest radar reports and other minor forecasting tools.

5th Step: Formulation Of The Forecast

       After the analysis of all available meteorological information/data has been completed, the preparation of forecasts follows.  The first and one of the preliminary steps is the determination as accurately as the data permit, of the location 24 hours hence of the different weather systems and the existing weather over a particular region.  In many cases a fairly satisfactory estimate of the direction and rate of movement may be made by simply measuring the movement during the last 12 or 24 hours and then extrapolating, or extending, this movement into the future and hence what weather will be experienced in different areas in the immediate future.


      The forecasts are then aired in various radio stations by telephone or sent by telefax machines a few minutes after completion and are immediately sent to the weather stations nationwide.  Copies are also furnished to different media outlets without delay.
 

Read more: How a Weather Forecast is Made


A

 

Absolute humidity:
is the weight of water vapor per unit volume of air.

Acid Precipitation:
Rain or snow with a pH value of less than 5.6; (sometimes caused by air pollutants.)

Adiabatic Temperature Change:
A cooling or heating of the air caused by the contraction or expansion of air molecules, as opposed to the loss or gain of heat. For example, adiabatic cooling takes place as air rises.

Advection:
Horizontal movement of air, moisture, or heat.

Advection Fog:
Horizontal movement of warm, humid air over colder ground or water.

Afterglow:
The glow in the western sky after sunset.

Air Mass:
A large body of air with nearly uniform temperature and moisture content.

Airstream:
A significant body of air flowing in the same general direction.

Altitude:
Height expressed as the distance above a reference point, which is normally sea level or ground level.

Aphelion:
the point in the path of a celestial body (as a planet)that is farthest from the sun.

Anemometer:
An instrument that measures wind speed.

Aneroid Barometer:
An instrument built around a metal structure that bends with changing air pressure. These changes are recorded on a pointer that moves back and forth across a printed scale.

Anisallobar:
isogram with the same rise of barometric pressure in a given time.

Anomaly:
The difference between the mean of any meteorological element, and the phase of that element over the same time for all other points on the same parallel of latitude.

Anticyclone:
a closed wind circulation of high barometric pressure that rotates clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. With respect to the relative direction of its rotation, it is the opposite of a cyclone.

Arctic Air:
A mass of very dry, very cold air that develops over the snow-and-ice-covered regions of the Far North.

Aridity:
The degree to which a climate lacks effective, life promoting moisture; the opposite of humidity.

Atmosphere:
The mass of air surrounding the Earth.

Atmospheric pressure:
The amount of force exerted on a unit surface area. Also called air pressure.

Aurora:
A luminous phenomenon that consists of streamers or arches of light and is caused by electrical discharges in the atmosphere, mostly confined in the tenuous air of high altitude. It is most commonly seen in sub-Arctic and sub-Antarctic latitudes. However, observations with the spectroscope seem to indicate that a faint permanent aurora is a normal feature of the sky in all parts of the world.

Aurora Australis:
An aurora that occurs in the southern hemisphere; also called southern lights.

Aurora Borealis:
An aurora that occurs in the northern hemisphere; also called northern lights.

Aviation weather forecast:
A forecast of weather elements of particular interest to aviation. These elements include the ceiling, visibility, upper winds, icing, turbulence, and types of precipitation and/or storms. It can be divided into four basic categories, area forecasts, terminal forecasts, route forecasts,and flight forecasts.

Aviation weather observation:
An evaluation, according to set procedure, of weather elements which are most important to aircraft operations. It includes the cloud height or vertical visibility, sky cover, visibility, obstructions to vision, certain atmospheric phenomena, and wind speed and direction that prevail at the time of the observation.

Azimuth:
The length of the arc of the horizon intercepted between a given point and an adopted reference direction, usually true north, and measured clockwise from the reference direction. It is a horizontal direction expressed in degrees. It is sometimes synonymous with bearing, but the latter is a navigation term and can be modified in several ways. Any point on or above the horizon can be located by its angles of azimuth and elevation plus either height or distance (or slant range) data.

B

Back-Door Cold Front:
A cold front that moves in from the northeast, rather than from the normal north or northwest direction.

Backing Wind:
Shifting of the wind in a counterclockwise direction, usually resulting from the approach of a low-pressure system.

Barograph:
An instrument that provides a continuous record of atmospheric pressure.

Barometer:
An instrument for measuring the pressure of the atmosphere. The two principal types are the mercury barometer and the aneroid barometer.

Barometric Tendency:
The amount and direction of change in barometer readings over a three-hour period.

Beaufort Wind Scale:
A system used to classify wind speed, indicated by numbers from 0 to 12. It was developed in 1805 by British Admiral Francis Beaufort.

Bishop ring:
A faint, broad, reddish-brown corona occasionally seen in dust clouds, especially those which result from violent volcanic eruptions. It has been seen after certain great volcanic eruptions, especially that of Krakatoa, in 1883.

Blizzard:
A violent, intensely strong cold wind, laden with snow.

Break:
A sudden change in the weather; usually applied to the end of an extended period of unusually hot, cold, wet, or dry weather.

Breeze:
A light wind with a speed ranging from 4 to 27 knots (4 to 31 mph or 6 to 50 kph).

Brine:
Sea water containing a higher concentration of dissolved salt than that normally found in the ocean. Brine is produced by evaporation or freezing of sea water.

Buys Ballot's law:
In the Northern Hemisphere, if you face the wind the atmospheric pressure decreases towards your right and increases towards your left. The reverse is true in the Southern Hemisphere. The law is useful in locating centers of cyclones and anticyclones.


C

Calm:
Absence of apparent motion of wind. This condition is reported when smoke is observed to rise vertically, or the surface of the sea is smooth and mirror-like.

Ceiling:
The height above the ground of the base of the lowest layer of clouds, when at least 60 percent of the sky is covered by clouds.

Centigrade:
A thermometric scale on which 0° denotes the freezing point and 100° the boiling point water, both under standard atmospheric pressure.

Circulation:
General circulation is the flow of air of large, semi-permanent weather systems, while secondary circulation is the flow of air of more temporary weather systems.

Climate:
An average portrait of weather conditions in a specific place over a long period.

Climatology:
The scientific study of climate.

Cloud:
A visible aggregate of minute water and/or ice particles in the atmosphere above the earth's surface.

Cloud banner:
A cloud streaming off from the mountain peak, resembling a banner.

Cloud-burst:
A sudden and extremely heavy downpour of rain; especially in mountain regions.

Col:
The point of intersection of trough and ridge in the pressure pattern of a weather map. A neck of relatively low pressure between two anticyclones; also called a saddle.

Cold Front:
The forward edge of an advancing cold air mass which is displacing warmer air in its path.

Cold Wave:
A rapid and marked fall of temperature during the cold season of the year over a short period of time.

Condensation:
The change of a substance from vapor to liquid, usually caused by a decrease in temperature of the substance; the opposite of evaporation.

Condensation Nuclei:
Small particles in the air around which water vapor condenses.

Conduction:
The transfer of heat by molecular action within a substance or when two substances are in direct contact.

Continental Air Mass:
An air mass that forms over land. it is usually dry, but may be cold or warm.

Continental Climate:
the type of climate characteristic of the interior of a continent. Compared to maritime climate, it has a large annual and daily range of temperature.

Contrail:
A cloud-like stream in cold, clear air formed behind the engines of an airplane.

Convection:
It is often used to indicate the vertical movement of warm air, as opposed to advection.

Convergence:
The condition that exists when the distribution of winds within a given area is such that there is a net horizontal inflow of air into the area. The removal of the resulting excess is accomplished by an upward movement of air; consequently areas of convergent winds are regions favorable to the occurrence of precipitation.

Coriolis Effect:
the curving motion of an moving object, such as air, caused by the rotation of the earth. In the northern hemisphere moving objects deflect to the right and in the southern hemisphere deflect to the left.

Corona:
A set of one or more colored rings, concentrically surrounding the disk of the sun, moon or other luminary when veiled by a thin cloud.

Cumuliform:
A general term applied to all clouds having dome-shaped upper surfaces which exhibit protuberances, bases of such clouds being generally horizontal. Cumuliform clouds are characteristically distinct and separated from one another by clear spaces.

Cyclone:
A low pressure system in which winds spin inward in a counterclockwise direction in the northern hemisphere.


D

Dangerous quadrant:
The advance quadrant of the dangerous semi-circle. A vessel and all its attendants in bad weather re-curve over it.

Dangerous semi-circle:
It is the half of the storm area in which rotary and progressive motions of the storm reinforce each other, and the winds are also directed in such a way as to drive a vessel running before the wind across the storm track ahead of the advancing center.

Deepening:
An area of low-pressure in which storm conditions occur.

Deviation of the Wind:
The angle between the direction of the wind and the direction of the pressure gradient.

Dew:
Atmospheric moisture condensed, in liquid form, upon objects cooler than the air, especially at night.

Dew point:
The temperature at which, under ordinary conditions, condensation begins in a cooling mass of air. It varies with the specific humidity. It is a conservative air mass property.

Disturbance:
An area of low-pressure in which storm conditions occur.

Diurnal:
Having a daily cycle, especially pertains to actions which are completed within twenty-four hours and which recur every twenty-four hours.

Diurnal Tide:
A tide in which there is only one high water level and one low water level on a lunar day.

Divergence:
The conditions that exists when the distribution of winds within a given area is such that there is a net horizontal flow of air outward from the region. The resulting deficit is compensated by a downward movement of air from above; consequently areas of divergent winds are regions unfavorable to the occurrence of precipitation.

Doldrums:
a part of the ocean near the equator abounding in calms, squalls, and light shifting winds.

Doppler Radar:
Sophisticated radar that can measure the speed and direction of moving objects, such as wind.

Downburst:
A sudden, strong, downward blast of air, usually from a thundercloud.

Drizzle:
Precipitation featuring tiny water droplets, no more than .02 inch in diameter. Unlike fog droplets, drizzle fall to the ground.

Drought:
Abnormally dry weather in a region over an extended period of time.

Dust:
Solid materials suspended in the atmosphere in the form of small irregular particles, many of which are microscopic in size.

Dry Fog:
A haze due to the presence of dust or smoke in the air.

Dynamic Meteorology:
The branch of meteorology that studies the motions of the winds and their relation to other atmospheric phenomena.

E

 

Earth hummock:
A small dome-shaped uplift of soil caused by the pressure of ground water. 

Earth shadow:
Any shadow projecting into a hazy atmosphere from mountain peaks during sunrise or sunset.
 
Easterly wave:
A migratory wave-like disturbance of the tropical easterlies. It is a wave within the broad easterly current and moves from east to west, generally more slowly than the current in which it is embedded. 

Eddy:
A more or less fully developed vortex in the atmosphere, constituting a local irregularity in a wind system. All winds near the earth's surface contains eddies, which at any given place produce gusts and lulls. Air containing numerous eddies is said to be turbulent

El Niño:
The occurrence of an unusually warm ocean current setting south along the coast of Ecuador, so-called because it generally develops just after Christmas. In exceptional years, concurrently with a southerly shift in the tropical rain belt, the current may extend along the coast of Peru to 12°S. 

Equinox:
When a line is drawn perpendicular to the plane of the earth's axis from the sun to the earth, the sun crosses the equator and day and night everywhere are of equal length. Vernal equinox occurs around March 21 (spring in the Northern Hemisphere) and Autumnal equinox occurs around September 22 (fall in the Northern Hemisphere). 

Evaporation:
The physical process by which a liquid or solid is transformed to vapor or the gaseous state; the opposite of condensation

Extratropical cyclone:
A cyclone that forms in the midlatitudes outside the tropics. 

Eye of the storm:
A calm region at the center of a tropical cyclone or a break in the clouds marking its location. 


F

Fahrenheit:
A thermometric scale on which 32° denotes the freezing point and 212° the boiling point of water, both under standard atmospheric pressure. 

Fair:
With respect to weather, generally descriptive of pleasant weather conditions, with regard to location and time of year. 

Fetch:
The area in which ocean waves are generated by the wind. It is generally delineated by coast lines,fronts, or areas of wind curvature or divergence. 

Flare:
A bright eruption from the sun's chromosphere (outer layers). Flares may appear within minutes and fade within an hour. They cover a wide range of intensity and size, and they tend to occur between sunspots or over their penumbrae. Also called solar flare

Flash Flood:
Flooding caused by a rapid rise in the water level of rivers, streams, or lakes, usually as a result of heavy rains. 

Foehn:
A warm, dry wind on the lee side of a mountain range, the warmth and dryness of air being due toadiabatic compression upon descending the mountain slopes.
 
Fog:
A cloud of water droplets suspended in the air that touches the ground; also described as a cloud at the earth's surface. 

Fog drip:
Moisture that is deposited on terrestrial objects by fog and then falls to the ground. 

Freezing:
The change in substance from a liquid to a solid state. 

Freezing Nuclei:
Particles suspended in the air around which ice crystals form. 

Freezing Rain:
Supercooled drops of water that turn to ice when they hit a cold surface.

Front:
The boundary between two different air masses. 

Frontogenesis:
The term used to describe the process which creates a front i.e., produces discontinuity in a continuous field of the meteorological elements; also applied to the process which increases the intensity of a pre-existing front. Frontogenesisis generally set up by the horizontal convergence of air currents possessing widely differing properties. 

Frost:
Ice crystals that form on grass and other objects when the temperature and dew point fall below freezing. 

Frost smoke:
A fog produced by apparent steaming of the sea in the presence of air having a temperature much below freezing. Also called Arctic sea smoke 

Fujita Scale:
A scale for estimating damage caused by the winds of tornado, developed by Theodore Fujita.


G

Gale:
An unusually strong wind, with velocities ranging from 28 to 47 knots (32 to 63 mph or 51 to 101 kph). In practice a wind of or exceeding force 8 on the Beaufort scale is counted a gale. 

Glaze:
Term applied to a smooth coating of ice on terrestrial objects due to the freezing of rain. In other countries such deposit is called glazed frost . A deposit of glaze on an extensive scale constitutes an ice storm

Gradient:
Change of value of a certain meteorological element per unit distance. The gradients commonly discussed in meteorology are the horizontal gradient of pressure, the vertical gradient of temperature, and the vertical gradient of electrical potential. Meteorologists now prefer the term lapse-rate to vertical gradient

Graupel:
Precipitation formed when water droplets freeze in layers around a falling ice crystal. Also called soft hail

Greenhouse effect:
the heating effect produced when the atmosphere absorbs and reemits infrared radiation. The shorter wavelength of insolation are transmitted freely through the atmosphere to be absorbed at the earth's surface, then earth reemits this as long-wave terrestrial radiation back to space. 

Gust:
A sudden brief increase in the force of the wind. It is of a more transient character than a squall and is followed by a lull or slackening in the wind speed. Most winds near the earth's surface display alternate gusts and lulls.

H

 

Hail:
Chunks of ice that form in layers in the updrafts of thunderstorms. 

Halo:
A ring or arc of light around the sun or moon caused by the refraction of light through ice crystals in the atmosphere.
 
Haze:
Particles of fine dust suspended in the air that produce limited visibility. 

Heat Index:
This index is a measure of the contribution that high humidity makes with abnormally high temperatures in reducing the body's ability to cool itself. 

Heat Lightning:
Lightning that can be seen, but too far away to be heard. 

Heat wave:
A period of abnormally high temperature. It is sometimes defined as a period of three or more consecutive days during which the maximum temperature is 90°F or over. 

Humidity:
The degree to which the air is charged with water vapor. This may be expressed as absolute humidity,relative humidity, and specific humidity

Hurricane:
A tropical cyclone in the western Atlantic that has sustained wind speeds of 74 miles per hour or greater.

Hydrometeor:
A generic term for weather phenomena that mostly depend upon modifications in the condition of the water vapor in the atmosphere. Examples are fograin and hail.
 
Hydrosphere:
The Earth's water. 

Hygrometer:
An instrument that measures the water vapor content of the air.


I

Ice:
The solid state of water; it is found in the atmosphere in different forms such as crystals, snow pellets etc.

Iceberg:
A large mass of ice that breaks from the tongue of a glacier, runs into the sea and then floats away. Only about ten percent of it is visible.

Iceblink:
A white, luminous appearance near the horizon caused by the reflection of light from ice.

Ice needles:
Thin crystals or shafts of ice, so light that they seem to be suspended in the air.

Ice rain:
A rain that causes a deposit of glaze. Falling pellets of clear ice.

Ice crystals:
Frozen water vapor suspended in the air.

Insolation:
Solar radiation that has been received by the earth's surface.

Instability:
A state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation.

Intertropical convergence zone:
The axis, or portion of the broad trade winds current of the tropics.This axis is the dividing line between the southeast trades and the northeast trades.

Inversion:
A condition in which air near the ground is cooler than air above it; a condition opposite to the concept of a normal decrease in temperature with height.

Isallobar:
An isogram having the same amount of change in barometric pressure within a specified period.

Isogram:
A line drawn on a chart or diagram to show the distribution of some physical condition in space or time (or both), by connecting points corresponding to equal values of the phenomenon represented. Most of the isograms used in meteorology are drawn on geographical charts, and show the distribution of meteorological elements in space only. Also called isoline.

Isanomal:
An isanomalous line; line of anomaly, i. e.,of the departure of the local mean value of an element from the mean pertaining to the latitude.

Isobar:
An isogram having the same barometric pressure for a certain period.

Isohyet:
An isogram having the same rainfall amount.

Isopleth:
An isogram that shows the variation of an element in relation to two coordinates; one of the coordinates representing the time of the year (month), and the other usually the time of the day (hour), and sometimes space (especially altitude).

Isotherm:
An isogram that surrounds an area with the same temperature.


J

Jetstream:
A narrow band of winds blowing high in the troposphere at speeds of 57 miles per hour or greater. 


K

Katisallobar:
An isogram having the same fall of barometric pressure in a given time. 

Kelvin:
A scale in which 0° is the point at which all molecular motion ceases (absolute zero). 

Kilopascal:
A metric unit of air pressure. It is simply millibars divided by 10. 


L

Land breeze:
The breeze that, on certain coasts and under certain conditions, blows from the land by night. 

Latent Heat:
The energy that is stored when water evaporates. The energy is released when water vapor condenses or is turned to ice.
 
Lifting:
The forcing of air in a vertical direction when it meets with an upslope in terrain or a denser air mass.
 
Lightning:
An electrical discharge produced by a thunderstorm. 

Line squal:
A more or less continuous line of squalls and thunderstorms marking the position of an advancing cold front. 

Low:
An area of low barometric pressure, with its attendant system of winds. An area of low barometric depression or cyclone.


M

March:
The variation of any meteorological element in the course of a day, year, or other interval of time. 

Maritime Air Mass:
An air mass that forms over water. It is usually humid, and may be cold or warm. 

Maritime Climate:
A type of climate characteristic of the ocean and oceanic islands. Its most prominent feature is a small range of minimum and maximum temperatures. 

Mean Temperature:
The average of a series of temperatures taken over a period of time, such as a day or a month. 

Mercury Barometer:
An instrument that measures atmospheric pressure by measuring the level of mercury in a column.
 
Mesoscale:
A large, rotating column of air that forms into a violent thunderstorm and may spawn tornadoes. 

Meteorology:
The science of the atmosphere. 

Meteor shower:
A large concentration of falling meteors. Many meteor showers are recurring phenomena and their appearance can be predicted. 

Microbarograph:
An instrument designed for recording small and rapid variations of atmospheric pressure.
 
Microbarometer:
An instrument used to show very small changes of atmospheric pressure

Microburst:
A downburst from a thunderstorm that is confined to a small area. 

Midlatitudes:
The areas in the northern and southern hemispheres between the tropics and the Arctic and Antarctic circles, the temperate zones

Millibar:
A metric unit of atmospheric pressure equal to 1/1000 bar or 1000 dynes (unit of force equal to the force that would give a free mass of one gram an acceleration of one centimeter per second per second) per square centimeter. 

Mirage:
An apparent displacement or distortion of observed objects by abnormal atmospheric refraction. Sometimes the images are inverted, magnified, multiplied, raised, or brought nearer to the eye than the object. Refraction layers in the atmosphere often assume the appearance of fog.

Mist:
A very thin fog in which the horizontal visibility is greater than 1 kilometer, or approximately 1,100 yards. Often used synonymously with drizzle or fine rain. 

Monsoon:
A wind that reverses its direction with the season, blowing more or less steadily from the interior of a continent toward the sea in winter, and in the opposite direction during summer.

N

 

Nautical mile:
The length of one minute of arc along any great circle on the earth's surface. Since this actual distance varies slightly with altitude, a nautical mile by international agreement is defined as 1852 meters (6076.103 feet or 1.1508 statue miles). 

Navigable semi-circle:
A vessel situated in this semi-circle may safely run before the wind, because she is then moving away from the storm. 

Neap tide:
A tide of minimum range, occurring twice a month one or two days after quadrature

Nephoscope:
An instrument for measuring the speed and movement of clouds.
 
Normal:
A numerical figure representing the average of conditions at a location over a period of years. 

Numerical Forecasting:
Forecasting the weather through digital computation carried by supercomputers


O

Oblique visual range:
The greatest distance at which a specific target can be perceived when viewed along a line of sight inclined to the horizon.
 
Occluded front:
The front that is formed when and where the cold front overtakes the warm front of the cyclone. Serves as a boundary between cold and warm air masses.
 
Occlusion:
The term used to denote the process whereby the air in the warm sector of a cyclone is forced from the surface to the higher levels. The process is accompanied by an increase in the intensity of the cyclone. 
Oceanography:
The study of the oceans, embracing and integrating all knowledge pertaining to the sea's physical boundaries, the chemistry and physics of sea water, and marine biology. 

Orographic lifting:
The upward flowing of air caused by rising terrain, such as a mountain range. 

Overcast:
Descriptive of sky cover of 1.0 (95 percent or more) when at least a portion of this amount is attributed to clouds or obscuring phenomena aloft; that is, when the total sky cover is not due entirely to surfaced-based obscuring phenomena. 

Overrunning:
the flow of warm air over cold air in advance of a warm front. 

Oxygen:
In its free form, a colorless, tasteless, and odorless gaseous element; the second most abundant gas in the earth's atmosphere and a prerequisite of virtually all forms of animal life. It is a usually a diatomic (containing two atoms) substance. 

Ozone:
A triatomic form of oxygen which occurs transiently in small quantities in the lower atmosphere and is supposed to be permanently present and relatively abundant at high atmospheric levels. 

Ozone layer:
otherwise known as ozonosphere; an atmospheric layer where there is a maximum concentration of ozone which blocks most solar ultraviolet radiation from entry into the lower atmosphere.


P

Path:
The direction in which the vortex will probably travel.
 
Perihelion:
The point in the path of a celestial body (as a planet) that is nearest to the sun.
 
Pilot balloon:
A small free balloon that drift off which, as observed from the ground, indicates the movements of the air aloft.
 
Polar Air Mass:
A mass of very cold, very dry air that forms in polar regions. 

Polar continental air mass:
Polar air mass that originates over land or frozen ocean areas in the polar regions. It is characterized by low temperatures, low specific humidity and a high degree of vertical visibility. 

Polar maritime air mass:
Polar air mass that originally came from the polar regions but has since been modified by reason of its passage over a relatively warm ocean surface. It is characterized by moderately low surface temperatures, moderately high surface specific humidity, and a considerable degree of vertical instability. 

Precipitation:
Any liquid or solid form of water that falls from the atmosphere and reaches the surface of the Earth, usually measured in millimeters. 

Pressure:
An elliptical expression, current in meteorological literature, for atmospheric pressure, or barometric pressure.
 
Pressure Gradient Force:
Force acting on air that causes it to move from areas of higher pressure to areas of lower pressure. 

Prevailing Winds:
The direction from which the wind blows frequently in any location.
 
Prevailing westerlies:
The belts of winds lying on the poleward sides of the subtropical high-pressure belts.
 
Psychrometer:
An instrument that measures relative humidity of the air. 

Pyrheliometer:
An instrument that measures solar radiation by its heating effects.


Q

Quadrature:
A configuration in which the moon and the sun have an angular separation of 90 degrees as seen from the earth. 


R

Radiation:
The transferring of energy through electromagnetic waves. 

Rain:
Liquid precipitation with drops larger than .02 inch in diameter.
 
Rainbow:
A luminous arc formed by the refraction and reflection of light in drops of water suspended in the atmosphere.
 
Rainfall:
A term sometimes synonymous with rain, but most frequently used in reference to amounts of precipitation which includes snow, hail, etc. 

Rain gauge:
An instrument for measuring rainfall. 

Reflection:
The production of an image by or as if by a mirror caused by light "bouncing"; off a certain surface. 

Refraction:
The bending of light as it passes through areas of different density, such as from air through ice crystals.

Relative humidity:
is the ratio of the actual vapor pressure to the vapor pressure corresponding to saturation at the prevailing temperature, or simply the percentage of saturation.
 
Ridge:
A relatively narrow extension of an anticyclone or high-pressure area as shown on a weather chart.
 
Right hand and left hand semi-circles:
The semi-circles are named on the assumption that the observer is at the storm's center, and, is looking along the path. 

Rime:
Tiny balls of ice formed when tiny drops of water freeze on contact with the surface.


S

Saturation:
The condition that exists in the atmosphere when the partial pressure exerted by the water vapor present is equal to the maximum vapor pressure possible at the prevailing temperature.
 
Sea breeze:
The breeze that, on certain coasts and under certain conditions, blows from the water; by day.
 
Season:
A division of the year according to some regularly recurring phenomena, usually astronomical or climatic.
 
Shear:
The variation of vector field along a given direction in space. The most frequent context for this concept iswind shear

Shower:
A snowfall or rainfall, of short duration but often of considerable intensity, and usually consisting of relatively large drops. Also a similar fall of sleet, or hail. Showers characteristically fall from the isolated clouds separated from one another by clear spaces. They occur typically in air masses that possess a high degree of instability. 

Sleet:
Precipitation consisting of ice particles formed when raindrops freeze. 

Smog:
Air pollution caused by a mixture of smoke and fog.
 
Snow:
Precipitation in the form of small ice crystals, falling either separately or in loosely coherent clusters (snowflakes).
 
Soft hail:
White, opaque, round pellets of snow. 

Solar constant radiation:
The intensity of solar radiation outside the Earth's atmosphere at the Earth's mean distance from the sun. Findings indicate that this intensity may vary and that its mean value is 1.94 gram-calories per minute per square centimeter of area lying normal to the incident solar ray.
 
Solar Energy:
The energy produced by the sun. 
Solstice:
The time of year when the sun is the farthest north or the farthest south (about June 21 and December 21). The time of year when there exists the largest difference between the hours of daytime and nighttime at certain latitudes. 

Sounding balloon:
A free, unmanned balloon carrying a set of self-registering meteorological instruments.
 
Specific humidity:
is the mass of water vapor contained in a unit mass of moist air.
 
Spring tide:
Tide near the time of syzygy, when ranges between high tide and low tide are greatest.
 
Squall:
A sudden storm of brief duration; closely akin to thunderstorm but not necessarily attended by thunder and lightning. A sudden brief blast of wind, of longer duration than a gust. 

Squall Line:
A line of thunderstorms that forms along a front. 

Stable Air:
Air in which temperature and humidity at various levels discourage the formation of convection currents. 

Stationary Front:
The border between cold and warm air masses that is not moving. 

St. Elmo's Fire:
A luminous brush discharge of electricity from elevated objects, such as the masts and yardarms of ships, lightning rods, steeples, etc., occurring in stormy weather. 

Storm:
A marked disturbance in the normal state of the atmosphere. The term is often applied to a disturbance in which strong wind is the most prominent characteristic, and specifically to a wind force of 11 on the Beaufort scale. It is also used for other types of disturbance, including thunderstorms, rainstorms, snowstorms, hailstorms, dust storms, sand storms, magnetic storms. etc. 

Storm Track:
The path that storms generally follow in a given time. 

Stratiform:
A general term applied to all clouds which are arranged in unbroken horizontal layers or sheets. 

Sublimation:
The change of water vapor directly into ice crystals or ice crystals directly into water vapor. 

Subsidence:
The word used to denote a slow downward motion of air over a large area. Subsidence accompanies divergence in the horizontal motion of the lower layers of the atmosphere. 

Summer solstice:
Occurs just a few days before aphelion. The sun is directly overhead at noon in latitude 23 1/2°N, and with regards to the amount of daylight hours, the Northern Hemisphere experiences the longest daylight hours and the Southern Hemisphere the shortest. Happens around June 21. 

Surge:
A general change in barometric pressure apparently ?superposed upon cyclonic and normal diurnal changes. 

Synoptic chart:
A chart, such as the ordinary weather map, which shows the distribution of meteorological conditions over an area at a given moment. 

Synoptic Meteorology:
The branch of meteorology that deals with the analysis of meteorological observations made simultaneously at a number of points in the atmosphere (at the ground or aloft) over the whole or a part of the earth, and the application of the analysis to weather forecasting and other problems. 

Syzygy:
The points in the moon's orbit about the earth at which the moon is new or full. At this time also the moon, earth and sun forms a nearly straight-line configuration. 


T

Thermometer:
An instrument for measuring temperature; in meteorology, generally the temperature of air. Maximum and minimum thermometers indicate, respectively, the highest and lowest temperatures occurring between the times of setting the instrument. A wet-bulb thermometer is used in measuring humidity. 

Temperate Zone:
The area of the globe between the tropics and the polar regions. 

Thunder:
The sound emitted or produced by a lightning discharge. 

Thunderstorm:
Weather disturbance that produces, aside from wind and rain,thunder and lightning. Thunderstorms occur locally, often as episodes of cyclones, and, in common with squalls, are marked by abrupt variations in pressure, temperature, and wind. 

Tidal Wave:
The wave motion of the tides. Any unusually high water level along a shore. It usually refers to either astorm surgeor tsunami

Tornado:
A violently rotating column of air that reaches from the base of a cloud to the ground (in funnel shape). In other areas it is called a violent thundersquall. 

Trace:
A precipitation amount of less than 0.005 inches. In general, an unmeasurable or insignificant small quantity. 

Track:
The direction from which the vortex or storm center has already moved. 

Trade wind:
Two belts of wind, one on either side of the equatorial doldrums in which the winds blow almost constantly from easterly quadrants. 

Tropical Cyclone:
The general term for a cyclone that originates over the tropical oceans. 

Tropical Depression:
A tropical cyclone with winds that do not exceed 63 kph. Most common in the region of the equatorial or intertropical convergence and less frequently in the trade winds. 

Tropical Storm:
A tropical cyclone with winds of 64 to 118 kph. 

Trough:
The elongated area of low barometric pressure that generally stretches north and south. This is the line where the lowest readings of the barometer will be recorded. 

True north:
The direction from any point on the earth's surface toward the geographic North Pole. 

Tsunami:
An ocean wave produced by a submarine earthquake, landslide, or volcanic eruption that may reach enormous dimensions and have sufficient energy to travel across entire oceans. 

Twilight:
Astronomical twilight is the interval between sunrise or sunset and the total darkness of night. Civil twilight is the period of time before sunrise or after sunset during which there is enough daylight for ordinary outdoor occupations. 

Typhoon:
A tropical cyclone with winds that exceed 118 kph. The name is applied to a severe tropical cyclone in the western Pacific.


U

Ulloa's ring:
A glory. A halo (also called Bouguer's halo), surrounding a point in the sky diametrically opposite the sun; sometimes described as a " white rainbow". 

Ultraviolet radiation:
Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays. 

Unstable Air:
Air with temperature differences that encourage the formation of convection currents that can produce clouds and precipitation. 

Updraft:
An upward current of air, usually within a thundercloud. 

Upwelling:
The rising of water toward the surface from subsurface layers of a body of water. 


V

Vane:
A device that shows which way the wind blows; also called weather vane or wind vane. 

Vapor pressure:
The pressure exerted by a vapor when it is confined in space. In meteorology vapor pressure refers exclusively to the pressure of water vapor. When several gases or vapors are mixed together in the same space each one exerts the same pressure as it would if the others were not present; the vapor pressure is that part of the total atmospheric pressure which is due to water vapor. 

Variability:
Inter-diurnal variability is the average difference between successive daily averages of a meteorological element. 

Veering Winds:
Winds that shift in a clockwise direction, a shift caused by a high-pressure system. 

Vernier:
An auxiliary scale for estimating fractions of a scale division when the reading to the nearest whole division on the main scale is not sufficiently accurate. 

Vertex:
The point of re-curvature, about 20° latitude. 

Visibility:
The maximum distance at which one can see and identify objects. 

Virga:
Water droplets or ice crystals that fall from high clouds but evaporate before hitting the ground. 

Vortex:
The central area of light winds and calms, or within the ring of hurricane force winds. 


W

Warm air mass:
An air mass that is warm in relation to its neighboring air masses. The term implies that the air mass originated in latitudes lower than those in which it now finds itself and that it is, therefore, warmer than the surface over which it is moving. 

Warm front:
The leading edge of a mass of warm air that displaces a mass of cold air. 

Warning:
A message given to relate the occurrence of a threatening weather condition, as indicated by a radar or spotter; or of one that is expected to occur or strike an area very soon. 

Waterspout:
A tornado-like vortex and cloud occurring over a body of water, frequently in tropical waters. 

Water vapor:
The invisible gaseous form of water. 

Wave:
A small cyclonic circulation in the early stages of development that moves along a cold front. 

Wave disturbance:
A localized deformation of a front, which travels along the front as a wave-shaped formation, which generally develops into a well-marked cyclone. 

Weather:
The conditions in the atmosphere at any given time. 

Weather forecast:
A forecast of the future state of the atmosphere with specific reference to one or more associated weather elements. 

Westerlies:
The dominant west-to-east motion of the atmosphere, centered over the middle latitudes of both hemispheres. 

Wind:
Air in motion that moves horizontally in relation to the surface of the Earth. Vertical streams of air are usually called currents

Windchill Factor:
A measure of the effect of wind in increasing the heat loss from exposed flesh. 

Wind Direction:
The direction from which the wind is blowing. 

Wind shear:
A sudden shift in wind direction. 

Winter solstice:
The Southern Hemisphere has its maximum exposure to the sun. It occurs just a few days before perihelion and the sun is directly overhead at noon in latitude 23 1/2°S. This occurrence brings about the shortest daylight hours in the Northern Hemisphere and the longest daylight hours in the Southern Hemisphere. Happens around December 21.


Z

Zenith distance:
The angular distance of any celestial object from a given observer's zenith, measured along the great circle of the celestial sphere from zenith to object; the compliment of the elevation angle. 

Zodiacal light:
A cone of faint light in the sky which is seen stretching along the zodiac from the western horizon after the twilight of sunset has faded and from the eastern horizon before the twilight of sunrise has begun. 

Read more: Meteorological Terms


INTRODUCTION

             
              Water, 
one of the most common substances known to man has become, through the years, a precious commodity. Water is required in practically all facets of human activities. And the need for water is enormous. It is roughly proportional to the population! However, the Philippines is blessed with a large number of rivers, lakes and streams. Thus, the lack of water has never been a real problem. Where an apparent scarcity exists, the difficulty is traceable to the uneven distribution of rainfall necessary to replenish water in rivers, lakes and other bodies of water. Paradoxically, it is sometimes excessive abundance of water that spells trouble. Because of the monsoons, the Philippines has a fairly well-defined wet season. In addition, there are other precipitation-producing weather phenomena: tropical cyclones, thunderstorms, the ITCZ, frontal passages,etc. Singly by themselves, these can generate large amounts of precipitation. In combination with each other or, in particular, with the monsoon, these phenomena are capable of bringing intense and excessive precipitation. Under certain conditions, a surfeit of rainfall results in a potentially disastrous phenomenon- flood. 

 

A layman's conception is often an over-simplification. The "Manual of Operational Procedures on Flood Forecasting and Warning" states:

"From a strict hydrological sense, flood is defined as a rise, usually brief, in the water level in a stream to a peak from which the water level recedes at a slower rate (UNESCO-WMO 1974). The episodic behavior of a river that may be considered flood is then termed "flood event" (Linsley, 1942) which is described as a flow of water in a stream constituting a distinct progressive rise, culminating in a crest, together with the recession that follows the crest (Linsley, 1942)."


          From the foregoing technical definition, flood simply denotes a progressive abnormal increase in the elevation of the surface level of streamfiow until it reaches a maximum height from which the level slowly drops to what is its normal level. The sequence described all takes place within a certain period of time.

       
          The definition merely describes a characteristic behavior. It does not include the element of "flooding" or inundation as implied by the popular notion of flood. The technical definition is rather inadequate. Thus, considering the intents and purposes of flood forecasting and warning, the definition seems rather restrictive in its connotation for the public.

     
         Hence, for operational purposes, the Flood Forecasting Branch, the hydrological service of PAGASA has adopted a more extensive definition. Flood is "an abnormal progressive rise in the water level of a stream that may result in the over­flowing by the water of the normal confines of the stream with the subsequent inundation of areas which are not normally submerged".

     
      The popular or layman's idea of flood is, in the strictest sense, the process of inundation or the coverage by water of areas not normally submerged. Inundation is due to water overflowing from streams and other bodies of water as well as by the accumulation of rainwater by drainage.

Floods are categorized into natural and artificial floods in terms of their specific causes.

Flood is basically a natural hydrological phenomenon. Its occurrence is usually the aftermath of meteorological events. These included:

  • an intense and prolonged rainfall spells;
  • unusually high coastal and estuarine waters due to storm surges, seiches, etc.



            Floods are also caused, indirectly, by seismic activities. Coastal areas are particularly susceptible to flooding due to tsunamis (seismic sea waves). Sinking of land due to earthquakes reduces the elevation of land areas. In the vicinity of lakes and rivers, these areas become flood-prone. Likewise, the uplifting of lake and river beds from seismic causes sometime results in the overflowing of these bodies of water. The water then inundates the surrounding and adjacent areas.

           To a certain extent, astronomically influenced phenomena such as high tides coinciding with the occurrence of heavy rainfall frequently cause flooding.
           Occasionally, floods occur unnaturally. These are usually the result of human activities. Such activities include:

  • Blasting - this causes landslides in the slopes of hills and mountains which may result in the unintentional damming of rivers and streams.
  • Construction of temporary dams - this produces an impediment to the flow of a river or stream which then results in an overflow;
  • Failure of hydraulic and other control structures - accidents like the breaking of a dike result in the entry of an enormous quantity of water in a protected area; and
  • Mismanagement of hydraulic structures - control structures like dams which are utilized for various purposes are usually operated according to what is known as an "operation rule" and mismanagement which results in the violation of the rule may necessitate an untimely and sudden release of large amounts of excess water.

 
          While not quite so obvious, human activities that tend to alter the ecological system in a river basin will have an impact on the hydrology of the catchment. This could, in the future, result in frequent floods. Foremost among such activities is the denudation of forest and watershed areas. Floods vary in degree of severity in terms of areas extent or magnitude and in depth. They are, thus, classified as minor or major flooding. In a minor flooding, inundation may or may not be due to overbanking. When there is no bank overflow, flooding is simply due to the accumulation of excessive surface run-off in low lying flat areas. Floodwaters are usually confined to the flood plain of the river along the channel, on random low-lying areas and depressions in the terrain. Floodwater is usually shallow and there may not be a perceptible flow.
         
         During a major flood, flooding is caused by the over­flowing of rivers and lakes; by serious breaks in dikes, levees, dams and other protective structures; by uncontrollable releases of impounded water in reservoirs and by the accumulation of excessive runoff. Floodwaters cover a wide contiguous area and spread rapidly to adjoining areas of relatively lower elevation. Flooding is relatively deep in most parts of the stricken areas. There is a highly perceptible current as the flood spreads to other areas.

        While floods take some time, usually from 12 to 24 hours or even longer, to develop after the occurrence of intense rainfall, there is a particular type which develops after no more than six hours and, frequently, after an even less time. These are what are known as "flash floods".

         Flash floods develop in hilly and mountainous terrains where the slope of the river is rather steep. The rapid development of the flood is due to the extremely short concentration time of the drainage catchment. This means that precipitation falling on a point in the catchment farthest from the river takes only a short time to reach the river channel and become part of streamflow. Thus, the amount of streamflow rapidly increases and, consequently, the rise in water level. When the flow capacity of the stream is exceeded, the channel overflows and the result is a flash flood.

Floods are among the most destructive calamities man has to cope with. Even the most minor flooding poses some inconveniences. A really big flood can result in millions even billions of pesos of damages to roads and bridges, buildings and other economic infrastructure, in the loss of agricultural crops and livestocks, loss of productivity in industry, commerce and trade. To this is added the incalculable loss of human lives directly attributable to floods as well as the hardship and attendant socioeconomic problems of forced human displacement and the emotional impact on those affected by floods.

    Aside from the direct damages brought by a flood there are also those cascading effects which follow in the wake of the calamity. Among the immediate problems caused by flood are the lack of basic utilities and essential necessities, particularly, food and potable water. Flood also disrupts the sanitary regime in a community. This almost always results in the contamination of the water supply. Thus, in a flood-stricken area an epidemic of gastro-intestinal diseases frequently breaks out. Respiratory ailments due to exposure are also quite common.  

        For obviously practical reasons, man has always preferred a riverine environment. Almost always naturally fertile and, therefore, able to provide him with an abundance of his needs, flood plains have always attracted man as a place to settle down. The river or a lake, in addition to being a source of his livelihood, also serves as a convenient means of communications, an avenue for trade and commerce.
 

       However, a river is not always placid, serenely flowing along. It has its moods. And it changes its moods, quite regularly. When it does, man comes face to face with the phenomenon of the flood.
 

       But man is also a resourceful creature. Knowing that he cannot prevent the recurrence of floods and because he has to live with it, man has learned to cope. He tried to understand the nature, behavior and the causes of floods.
 

       In some primitive ways, he had learned to relate the vagaries of the weather to the behavior of the river. In doing so, he established a "forecasting procedure" which has persisted up to our times and which remains practically unchanged.
 

       Modern flood forecasting is now based on the standard procedure of monitoring and analysing the hydrological and meteorological conditions in a river basin. While the tools and methods of monitoring may have been modernized with the use of sensitive, telemeterized gauging instruments to effect better observation and faster transmission of data, it is still basically an attempt to paint a bread picture of what is currently happening, hydrologically and meteorologically, in a river basin.
 

      The simple method of associating the weather to the behaviour of the river has given way to the more modern sophisticated analytical methods aided by the computerized flood forecasting models. With further evaluation of the results of the various analysis, hydrologists are able to come up with a prediction of the future state of the river.
 

      If anything new is added to modern flood forecasting, it is the more extensive reliance of hydrologists on the scientific principles of hydrology rather than on mere intuition.

         The preparation, issuance and dissemination of an adequate and timely warning is the ultimate purpose of flood forecasting. Timeliness is an essential requirement for a flood warning. A sufficient lead time enables the ultimate user to take the necessary precautionary countermeasures.
 

         The hydrological and meteorological conditions in a river basin and the consequent state of its river system is never constant. The behavior of the river itself is the resultant of the interaction of all hydrological processes and conditions in the river basin.
 

         A flooding situation is not a daily occurrence. However, flood forecasting operations must, of necessity, be a continuous activity. It is carried out from day to day even when the possibility of a flood is highly improbable. This mode of operation enables flood forecasters to pinpoint the beginning of a potential flood-generating situation.
 

        Like storm bulletins which are issued only during the presence of tropical cyclones, flood forecast and warning bulletins are prepared only when a potential flooding situation is definitely present. They are issued regularly at specified hours of the day for the duration of the flooding period until the flood recedes or when all hazards and dangers associated with the phenomenon are no longer present.
 

       The prevailing hydrometeorological situation in a given river basin defines the operational environment under which flood forecasting and warning operations are carried out. Conveniently categorized into
 

  • normal situation,
  • alert phase, and
  • warning phase

these operational situations are dependent for their implementation on the behavior and state of the river with respect to a set of criteria known as flood assessment levels.

      In practice1 flood bulletins are issued as soon as the development of a flooding situation exists. The initial bulletin serves merely to alert the people in the threatened basin to the possibility of a flood. It is never intended to categorically state that there will be flooding but only of the possibility. The initial bulletin is issued as soon as the operational situation passes from the alert to the warning phase. The transition from one operatiohal status to another is based on pre-determined criteria.

      Thus, flood forecasting operation needs to be a continuing activity to determine the point in time when the operational situation passes from one phase to another. A flood forecasting operation consists of the following:

 

Monitoring:

This requires the collection, at regular interval, of the real-time data on rainfall, water level and other information that affect the hydrological condition of the river basin and the state of the river system. This provides a broad picture of the current situation in a river basin.

Analysis:


The data are analysed and related to other available information such as storm data from radar and satellite observation. The general objective here is to deduce the probable development in the hydrological situation in the river basin in the near future. This part of the operation involves a variety of hydrological analyses as well as the use of flood forecasting models to provide an objective estimate of the fore­cast situation.
 

 

 

 

Preparation of the forecast and warning:


Upon receipt of the coded messages, they are decoded and each set of observations is plotted in symbols or numbers on weather charts over the respective areas or regions.  Observations made over land and sea are plotted on the surface or mean sea level charts which are prepared four times a day.  Radiosonde, theodolite, aircraft and satellite wind observations are plotted on upper level charts which are prepared twice daily.

 

Dissemination of the flood forecast and warning:

Flood forecasts are completed in time for release at regular preset time of issuance. Dissemination is made through dissemi­nating agencies such as the OCD1 DSWD, NDCC, and thru the mass media, particularly radio and television.

 

There are three categories of flood information intended for the general public. All are issued under the general title of "Flood Bulletin". A distinct series of bulletins is issued for each threatened river basin where the forecasting and warning service is already extended and operational. As already stated, bulletins are issued only when conditions,

i.e., there is a potential flooding situation, so warrant their issuance.

      When required, flood bulletins are prepared twice daily. They are completed and readied for issuance and dissemination at 5 a.rn. and 5 p.rn., respectively, when it is deemed early enough to provide vital information for concerned users to take necessary countermeasures before they leave for work in the morning or before they retire at night.

      A series of bulletins for a given affected river basin is ideally initiated by a Flood Outlook. As the category implies, the bulletin merely states the present hydrological situation and alerts the people in a basin to the possibility of a deteriorating condition, e.g., a gradual and continuous rise in the water level.


     Subsequent bulletins could be of any one of the three categories. Normally, for a given affected basin, one bulletin is followed by another of the same in the next higher category depending on the development in the hydrological condition and the forecast situation. Hence, an Outlook is followed by another Outlook or by a Flood Advisory; or by a Flood Warning.

     When the situation had clearly reached its worst such that, at most the condition or, at best, improvement can be expected, subsequent bulletins are of the same or by a next lower category than the proceeding.

A "Flood Advisory" is a warning that states the imminence of a flood situation. Thus, it also contains suggested necessary actions that may have to be taken by the residents and the community in the threatened basin. An advisory is issued when the hydrological situation deteriorates further. It is also issued when condition is definitely improving but caution is still necessary.

A "Flood Warning" is issued when a flooding situation is a definite reality at least 24 hours before actual flooding occurs. This category is maintained in succeeding bulletins as long as the affected areas are inundated and the attendant dangers are present. Aside from the forecast, a warning states the necessary precautionary measures and actions residents as well as the affected community must take.

Flood bulletins are specifically directed to the public. They are intended to apprise the people in the threatened area of the present situation and of the expected development. It suggests the appropriate actions the community may have to take to prevent or mitigate the disastrous effects of a flood.

    As in any kind of disaster, the best countermeasures for flood damage prevention and mitigation are those which are community efforts.

    Floods cannot be prevented. To a large extent, however, they can be controlled effectively. By this is meant keeping the river from overflowing. There are a number of ways of accomplishing this before and during a flood:

  • increasing the flow capacity of a river by cleaning the channel of debris, by dredging, by straightening of channels, etc.;
  • construction of dikes and levees; and
  • sandbagging during floods.

    When overbanking can no longer be avoided, flood control can take the form of directing floodwaters where it can do the least damage.

    Individually and collectively, people in a flood-stricken area must take precautionary measures to ensure personnel safety and health:

  1. People, particularly children, should avoid wading in floodwaters.
  2. Where houses are expected to be flooded, people should move to higher places.
  3. Electrically operated appliances should be transferred to upper storeys of buildings.
  4. When electrical fines and outlets will be submerged in floodwater, power should be switched off.

    Flood damage mitigation and protection is a concern not only during the disaster. It should be practiced before, during and after the occurrence of a flood.   


Flood bulletins are specifically directed to the public. They are intended to apprise the people in the threatened area of the present situation and of the expected development. It suggests the appropriate actions the community may have to take to prevent or mitigate the disastrous effects of a flood. 


    As in any kind of disaster, the best countermeasures for flood damage prevention and mitigation are those which are community efforts. 

    Floods cannot be prevented. To a large extent, however, they can be controlled effectively. By this is meant keeping the river from overflowing. There are a number of ways of accomplishing this before and during a flood:

  1. increasing the flow capacity of a river by cleaning the channel of debris, by dredging, by straightening of channels, etc.;
  2. construction of dikes and levees; and
  3. sandbagging during floods.

    When overbanking can no longer be avoided, flood control can take the form of directing floodwaters where it can do the least damage. 

    Individually and collectively, people in a flood-stricken area must take precautionary measures to ensure personnel safety and health:

  1. People, particularly children, should avoid wading in floodwaters.
  2. Where houses are expected to be flooded, people should move to higher places.
  3. Electrically operated appliances should be transferred to upper storeys of buildings.
  4. When electrical fines and outlets will be submerged in floodwater, power should be switched off.

    Flood damage mitigation and protection is a concern not only during the disaster. It should be practiced before, during and after the occurrence of a flood.

 

    

BEFORE THE FLOOD:

  • Find out how often your location is likely to be flooded.
  • Know the flood warning system in your community and be sure your family knows it.
  • Keep informed of daily weather condition.
  • Designate an evacuation area for the family and livestock.
  • Assign family members instructions and responsibilities according to an evacuation plan.
  • Keep a stock of food which requires little cooking and refrigeration; electric power may be interrupted.
  • Keep a transistorized radio and flashlight with spare batteries, emergency cooking equipment, candies, matches and first aid kit handy in case of emergency.
  • Store supplies and other household effects above expected flood water level.
  • Securely anchor weak dwellings and items.
WHEN WARNED OF FLOOD:
  • Watch for rapidly rising flood waters.
  • Listen to your radio for emergency instructions.
  • If you find it necessary to evacuate, move to a safe area before access is cut off by flood waters.
  • Store drinking water in containers, water service may be interrupted.
  • Move household belongings to upper levels.
  • Get livestock to higher ground.
  • Turn off electricity at the main switch in the building before evacuating and also lock your house.
DURING THE FLOOD:
  • Avoid areas subject to sudden flooding.
  • Do not attempt to cross rivers of flowing streams where water is above the knee.
  • Beware of water-covered roads and bridges.
  • Avoid unnecessary exposure to the elements.
  • Do not go swimming or boating in swollen rivers.
  • Eat only well-cooked food. Protect leftovers against contamination.
  • Drink clean or preferably boiled water ONLY.
AFTER THE FLOOD:
  • Re-enter the dwellings with caution using flashlights, not lanterns or torchers. Flammables may be inside.
  • Be alert for fire hazards like broken wires.
  • Do not eat food and drink water until they have been checked for flood water contamination.
  • Report broken utility lines (electricity, water, gas and telephone) to appropriate agencies authorities.
  • Do not turn on the main switch or use appliances and other equipment until they have been checked by a competent electrician.
  • Consult health authorities for immunization requirements.
  • Do not go in disaster areas. Your presence might hamper rescue and other emergency operations

Flood are aggravated by factors resulting from the carelessness and indifference of people usually before floods occur.

THINGS ONE CAN DO TO MITIGATE FLOODS:

  • Regulate cutting of trees.
  • Report illegal loggers and kaingeros.
  • Report illegal construction of fishponds and other establishments in waterways.
  • Do not throw garbage in esteros and rivers.
  • Help clean the neighborhood.
  • Support community activities intended to lessen the occurrence of floods.
  • Avoid throwing anything like plastic wrappers anywhere which may clog or block the drainage system.


It is easy to say, and admittedly, floods have a tremendous socio-economic impact. Its main effect is to retard development. A flood-stricken area must first be restored to normal before any development activity can be carried out. Restoration can take time. The social and emotional trauma inflicted on the people usually have a short-term inhibiting effect on the community's drive. Hence, a little time can elapse before any concerted move for normalization can take place.


Flood damage is incalculable. Assessment of damage attributable to floods alone is difficult. Floods usually occur in association with other natural destructive phenomena such as tropical cyclones. Except in rare cases such as the Angat River disaster of the late 70's where loss of lives and property is identifiably due to the flood alone, it is difficult to segregate damages caused by a flood and those which resulted from the associated phenomenon. Therefore, precise quantifiable damage is always difficult to estimate. In addition to the directly determinable losses may be added the indirect potential losses. These results from unproductivity in many areas - in business, in trade, in commerce, etc. All these losses can wipe out whatever gains that may have been achieved in economic development.



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Thermometers

thermometerA thermometer measures the degree of hotness or coldness of a given substance. It operates on the principle of thermal expansion of the material used, e.g., liquids like mercury and alcohol, metallic materials, etc. Mercury is one of the liquids very sensitive to changes of temperature. When the substance to be measured is warm, mercury expands and rises in the capillary tube. When it cools, mercury contracts.

Maximum-Minimum Thermometer

minimum-maximum thermometerIn order to measure the temperature range, a set of maximum and minimum thermometers are used. A maximum thermometer has a constriction above the bulb that permits the mercury to rise in the capillary tube but does not allow it to descend the capillary tube unless the thermometer is reset. The highest point that the mercury reaches indicates the maximum temperature for the period. The minimum thermometer, on the other hand, gives the lowest temperature. It uses colored alcohol (because of its low freezing point). It is placed at an angle of about 20° from the horizontal. The black float called index needle is pulled downslope to the lowest temperature of the day by two forces; a) the surface tension at the top of the alcohol column and b) the force of gravity.

Thermograph

thermographA thermograph is an instrument that records air temperature continuously on graphing paper. It usually consists of a cylinder made to revolve once each week by means of clockworks inside. A sheet of graph paper is fastened on the outside. A pen-point that rests on the paper traces the temperature curve, according to the expansion and contraction of a sensitive metallic coil or strip corresponding to the reading of a thermometer.

These instruments are housed in a thermometer shelter which has double-louvered sides and a double-top roofing designed to permit air to circulate freely through the shelter.




Mercurial Barometer

mercurial barometerA mercurial barometer is a simple barometer made by filling a glass tube 32 inches long with mercury and inverting it so that the open end of the tube is below the surface of mercury in a cistern. The height of the mercury column is measured by sliding a vernier attached on a scale. To obtain accurate measurements, corrections are made for temperature expansion of the instrument, gravity and latitude. Values are read in millibars, millimeters or inches of mercury.

Aneroid Barometer

aneroid barometerAn aneroid barometer is made by removing the air from a thin, circular, metallic box. With practically no air on the inside the box would collapse. A spring is installed to limit the collapse of the box commensurate to the air pressure or weight of the column of air on the box. If one side of the box is fixed, the other side will move due to changes in atmospheric pressure. The surface of the metallic box is corrugated in order for the box to collapse and return uniformly. The movement of the spring causes a pointer to move over a scale of figure corresponding to the readings of a mercury barometer.

Since air pressure decreases with increase in altitude, the aneroid is also used as altimeters. On the altimeter, the scale is marked off in hundreds and thousands of feet or meters above sea level. The altimeters is a basic instrument in aeronautical stations and on board an aircraft.

Barograph

barographA barograph is a recording barometer. The pen point that traces the pressure curve on the paper is made to move up or down by means of a series of levers attached to aneroid cells (metallic boxes) in tandem. The use of aneroid cells in tandem provide a more pronounced response to changes in atmospheric pressure than would be indicated by a single aneroid cell of the same size.



Sling Psychrometer

sling psychrometerThe sling psychrometer consists of a dry and wet-bulb thermometer. The term bulb refers to that portion of the glass tube where the mercury is stored. The dry and wet bulbs are exactly alike in construction. The only difference is that the wet-bulb has a piece of muslin cloth or wick wrapped around its bulb and which is dipped in water shortly before the psychrometer is read.

This is how it is done. The weather observer firsts wets the cloth cladding the wet-bulb, whirls the psychrometer a few times, then reads the wet-bulb. He reads the dry-bulb last. Normally, the wet-bulb's reading will be lower than the dry-bulb's. The dry-bulb reading is the air temperature. The difference between the dry and the wet-bulb readings will give, with the aid of a psychrometric table, the dew point temperature and the relative humidity. (Dew point is the temperature at which the water vapor will condense while relative humidity is the ratio of the amount of water vapor actually present in the air to the maximum amount of water vapor the air can hold at a given temperature).

Hygrometer

hygrometerThe other instrument used to measure humidity is the Hygrometer. The hygrometer is less accurate than the psychrometer. It uses human hair from which the oil has been removed by using ether. The hair becomes longer as the relative humidity of the air increases. This change can be made to move an indicator needle which moves over a scale, the graduations of which reads from 0% to 100%.

Hygrothermograph

hygrothermographThe hygrothermograph records both relative humidity and temperature on graph paper in the same manner as the thermograph and barograph do.



8-inch Raingauge

8-inch raingaugeAn 8-INCH RAINGAUGE, so called because the inside diameter of the collector is exactly 8 inches above a funnel that conducts rain into a cylindrical measuring tube or receiver. The volume of the collector is 10 times the volume of the measuring tube. Therefore the actual depth of rainfall is increased ten times on being collected in the smaller measuring tube.

To measure the amount of rainfall accumulated in the measuring tube, (a) a thin measuring stick with the magnified scale printed on its face is used. The precisely dimensioned (b) measuring tube has a capacity representative of only 2 inches (50.8 millimeters) on flat level ground. Rainfall exceeding this amount spills into the (d) overflow can but can be easily measured by pouring it into the measuring tube for total rainfall.

Tipping Bucket Raingauge

tipping bucketThe tipping-bucket raingauge is a type of rainfall recording instrument. It is an upright cylinder that has funnel-shaped collector. The precipitation collected by the collector empties into one side of a "tipping bucket", an inverted triangular contraption partitioned transversely at its center, and is pivoted about a horizontal axis. Once one compartment is filled with rain, it tips, spilling out the water and placing the other half of the bucket under the funnel. The tipping activates a mercury switch causing an electrical current to move the pen in the recorder. Each tipping is equal to one-half millimeter of rainfall.



Ceiling Light Projector

ceiling projectorA ceiling light projector projects vertically a narrow beam of light on to a cloud base. The height of the cloud base is determined by using a clinometer located at a known distance from the projector to measure the elevation angle included by the illuminated spot on the cloud, the observe, and the projector. From trigonometry, the height of the cloud base is equal to the distance of the observer from the ceiling light projector multiplied by the tangent of the elevation angle.

Ceiling Balloon

ceiling balloonAnother way of determining the height of the cloud base is by using a ceiling balloon. A ceiling balloon is a meteorological balloon whose rate of ascent has been predetermined. It is filled with gas lighter than air, usually hydrogen, and released. The time of release and the time the balloon disappears into the cloud are recorded. The time difference multiplied by the rate of ascent will give the height of the cloud base.



Pilot Balloon/Theodolite

theodoliteA Pilot Balloon is a meteorological balloon that is filled with gas lighter than air. When the pilot balloon is used in conjunction with a theodolite it is used to determine the speed and direction of winds at different levels of the atmosphere.

The theodolite is similar to an engineer's transit. It consists of a sighting telescope mounted so that it is free to rotate around a horizontal and a vertical axis and has graduated scales so that the angles of rotation maybe measured while tracking the pilot balloon.

The elevation angles and azimuths of the balloon are recorded from the theodolite and these data at the end of the flight which may last for more than an hour are plotted to a plotting board. The wind speed and direction at selected levels are calculated either by trigonometric methods or graphical methods.

Night observation is accomplished by attaching a lit paper lantern to the balloon.

Radiosonde

radiosondeRadiosonde, an airborne instrument used for measuring pressure, temperature and relative humidity in the upper air is the radiosonde. he instrument is carried aloft by a meteorological balloon inflated with hydrogen. The radiosonde has a built-in high frequency transmitter that transmits data from the radiosonde meter and recorded on the ground by a specially designed radiosonde receiver.

Rawinsonde

radiosondeA more sophisticated version of this instrument is the rawindsonde. The rawindsonde is an electronic device used for measuring wind velocity, pressure, temperature and humidity aloft. It is also attached to a balloon and as it rises through the atmosphere, it makes the required measurements.

Rawin

Another special instrument is the Rawin which is short for Radar and Wind. It is an electronic device that measures pressure, temperature and humidity.

Wind Finding Radar

Another instrument is the Wind Finding Radar. It determines the speed and direction of winds aloft by means of radar echoes. A radar target is attached to a balloon and it is this target that is tracked by ground radar. The bearing and time of interval of the echoes is evaluated by a receiver.

Weather Surveillance Radar

A Weather Surveillance Radar is of the long range type which detects and tracks typhoons and cloud masses at distance of 400 kilometers or less. This radar has a rotating antenna disk preferably mounted on top of a building free from any physical obstruction. Radio energy emitted by the transmitter and focused by the antenna shoots outward through the atmosphere in a narrow beam. The cloud mass, whether it is part of a typhoon or not, reflects a small fraction of the energy back to the antenna. This reflected energy is amplified and displayed visually on a radar scope. The distance or slant range of the target from the radar is determined through the elapsed time the signal is transmitted and then received as an echo. Its direction is determined by the direction at which the focused beam is pointing at the instant the echo is received. The radar is a useful tool in tracking and monitoring tropical cyclones.













Read more: Weather Instruments




Introduction


A cloud is a visible mass of tiny particles of water or ice, or a mixture of both, suspended in the air.

Cloud particles are about 5 to 75 micrometers (0.0005 to 0.008 cm/0.0002 to 0.003 in) in size, which explains why light, vertical currents can sustain them in the air.

Clouds are formed when water vapor, which is an invisible gas, condenses and turns into a liquid. This liquid collects on particles such as dust in the air (called condensation nuclei) and become visible. Factors that determine when condensation may occur are air temperature, air pressure at a given altitude, the movement of the air and its moisture content. 

Cloud formation can occur at warm and cold fronts, at the side of a mountain where air flows up and then cools as it rises higher into the atmosphere (called orographic uplift), and when warm air blows over a colder surface such as a cool body of water.

The classification of clouds was first conceptualized by French naturalist Jean Lamarck in 1801. Two years later, in 1803, the English scientist Luke Howard created a classification which was later adopted by the International Meteorological Commission in 1929. 

The first scientific study of clouds began in 1803, when a method of cloud classification was devised by the British meteorologist Luke Howard. The next development was the publication in 1887 of a classification system that later formed the basis for the noted International Cloud Atlas (1896). This atlas, considerably revised and modified through the years (most recently in 1956), is now used throughout the world.

Clouds are generally classified according to genera in which Latin words are used to describe the appearance of clouds as seen by an observer on the ground. The table below summarizes the four principal components of this classification system.

Latin Root cumulus stratus cirrus nimbus
Translation heap layer curl of hair rain

Clouds are further categorized according to their height above the ground (etages). These are:

  • High-Level Clouds
    Forms above 20,000 feet (6,000 meters) and are primarily composed of ice crystals. Denoted by the prefix cirro- or cirrus and includes cirrus, cirrocumulus, and cirrostratus.
  • Mid-Level Clouds
    Their bases appear between 6,500 to 20,000 feet (2,000 to 6,000 meters). Composed primarily of water droplets although they can also be composed of ice crystals when temperatures are cold enough. Denoted by the prefix alto- and includes altostratus, altocumulus and nimbostratus.
  • Low-level Clouds
    Their bases generally lie below 6,500 feet (2,000 meters). Mostly composed of water droplets but may also contain ice particles and snow. Includes stratus, stratocumulus, cumulus and cumulonimbus.


High Clouds

Cirrus
Detached clouds that are delicate and fibrous in appearance, generally white in color, often resembles hooks or tufts, and are arranged in narrow bands. Also know as mares' tails.

Cirrus
Cirrocumulus

Cirrocumulus
Also called mackerel sky, it appears as thin, white patches, sheets or layers arranged like small flakes, granules, or ripples.

Cirrostratus
Thin, whitish veil of fibrous or smooth appearance, totally or partly covering the sky, frequently producing halo phenomena.

Cirrostratus

Middle Clouds

Altocumulus
Characterized by patches or layers of cloud, composed of rounded masses or rolls, which may be partly fibrous.

Altocumulus
Altostratus

Altostratus
Sheet or layer of cloud that is greyish or bluish in color, appears fibrous or uniform. The sun or moon may be seen only diffusely, as through ground glass. Does not show halo phenomena.

Nimbostratus
A dark grey cloud layer, diffused in appearance and usually producing continuously falling rain or snow. It is thick enough throughout to blot out the sun.

Nimbostratus

Low Clouds

Stratocumulus
Grey or whitish, patches or layers of cloud with dark parts, composed of rounded masses or rolls which frequently cover the sky. They are non-fibrous in appearance.

Stratocumulus
Stratus

Stratus
Uniform grey or white cloud layer that looks like fog aboveground. Produces drizzle, ice prisms or snow grains and may appear in the form of ragged patches.

Cumulus
Thick, detached clouds, in the form of rising mounds, domes or towers, with a flat, horizontal base and the upper part looking like a cauliflower. Usually associated with fair weather.

Cumulus
Cumulonimbus

Cumulonimbus
Dark, heavy and dense clouds, rising in the form of mountains or huge towers with its top shaped like an anvil. Produces thunderstorms, which is why it is also known as the thunderstorm cloud or thunderheads.

 

 

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INTRODUCTION
Giant bodies of air called air masses are always moving and bumping into each other.  Cold air and warm air do not mix easily because they have different densities.  Instead, a boundary forms between these air masses.  This bounday which separates the two air masses is called a front

An air mass is a large volume of air that is relatively uniform (horizontally) in temperature and water vapor concentration over hundreds of kilometers.  Air masses are generally identified with the regions over which they develop.  Two examples are continental polar and maritime tropical air masses.  While air masses can persist over their formative regions for a considerable length of time, they often move across regions.  As air masses move from one region to another, the air mass characteristics are modified by the underlying suface.  For instance, as cold, dry "arctic air" moves over an ocean surface it gains heat and moisture.

Major weather changes occur along the boundaries between the different air masses.  In reality, these boundaries are transition zones that for practical purposes can be considered as discontinuities or frontal surfaces.  Owing to the general circulation of the air, the frontal surfaces are in equilibrium whenever they form a small angle with the horizontal.  A frontal surface may therefore be defined as a transition-boundary separating two air masses of contrasting properties (temperature and humidity).  Simply put, fronts form when one kind of air mass enters an area occupied by another kind of air mass.


Fronts can be classified geographically and also according to the motion of the air masses involved. 

Geographical Classification

The formation of fronts occurs from time to time in widely different regions over the earth's surface.  However, it follows from what has been said about frontogenesis that the development of fronts occurs most frequently in certain geographical regions, particularly, where deformation fields prevail with suitable temperature gradient.  Regions favorable to frontogenesis are usually found along the boundaries of the four major air masses.  The fronts along these boundaries are called artic, polar, and intertropical front according to their mean positions.

Classification According To Motion Of Air

This classification is based primarily on the displacement of fronts and the resultant temperature changes.  Four basic types are recognized by this classification: cold front, warm front, occluded front and stationary front.

cold front is the boundary along the leading edge of a cold air mass that is pushing out a warm air mass.  As the cold front nears your region, the barometer falls.  The cold air behind the front wedges under the warm air and lifts it sharply off the ground.  Large cumulonimbus clouds appear.  These clouds often bring thunderstorms and rain showers.  As the cold front passes, the wind changes direction.   The weather becomes clear and colder and the barometer rises again


Further differentiation of cold fronts on the basis of vertical velocities is shown in two types.   Type I cold fronts are those observed outside the zone of cyclonic activity and are generally slow moving which may even be quasi-stationary in some cases.  It is defined by a general upglide of warm air over the entire frontal surface so that the cloud system resembles that of a warm front.   Type IIcold fronts are observed within the zone of cyclonic activity and as a rule move very rapidly.  Warm air is lifted only along the leading edge of the intruding wedge of cold air.  At higher levels, the warm air is moving faster than the cold air and therefore no ascending motions are observed in the warm air above the frontal surface.


warm front is the boundary along a warm air mass that is pushing out a cold air mass.  The warm air behind the front glides up and over the cold air. As a warm front approaches, high cirrus clouds appear.  These are followed by stratus and nimbostratus clouds.   The barometer falls and a long, steady rain begins. Gradually, the front passes and the sky clears.  Temperature rises as warm air replaces cold air, and the barometer stops falling.


stationary front forms when two air masses remain over a region for several days.  The front formed does not move.


An occluded front forms when a cold front overtakes a slower-moving warm front.  The occluded front is more complicated than the others because two fronts interact.  In the left diagram, colder air is wedging under warm air at the cold front.  Warm air is gliding up and over another cold air mass at the warm front.  The result of what is happening in the left diagram is seen in the right diagram.  The arm air squeezed out and lifted above the ground.  Steady rains falls at an occluded front.




During the winter months in the northern hemisphere, cold fronts usually extend to as far as the southern portion of the Philippines.  They are responsible for some of the rainfall along the eastern coasts of the country. 

For forecasting purposes, it is not sufficient to know whether one is dealing with a cold front or a warm front.  In order to forecast the weather conditions accurately, it is further necessary to know the stability conditions in both air masses as well as the distribution of vertical velocities.  An estimate of the stability of the respective cold and warm air masses can usually be gained from upper-air sounding and cloud observations.  The determination of vertical currents on a large scale is not feasible so far; however, an estimate of the sign of the vertical velocity can usually be obtained from the distribution of hydrometeors and clouds.


Pressure Distribution.
     Active warm fronts can generally be located by pressure troughs on the surface charts.   Since the troughs are seldom pronounced as those observed with cold fronts, other meteorological elements are resorted to in locating warm fronts accurately.

Pressure Tendencies.     Pressure usually falls for an appreciable length of time prior to the frontal passage.  Advection of warmer air and divergence above the frontal surface, and the removal of colder air in the lower layers account for the falling tendency.  Extensive negative isallobaric patterns precede warm fronts and roughly coincide with the warm-front cloud shield.  A marked decrease in the isallobaric gradient is observed in the warm sector except when rapid deepening of the associated cyclone is taking place. 

Wind.     The wind velocity usually increases in advance of warm fronts because of the general increase in the pressure gradient.  Gustiness, which is noticeably absent in the underlying cold air, has been observed to increase below the warm-front altostratus deck, particularly during the cold season of the year over the continents and during the warm season over the oceans.  The approach of the front is usually marked by increasing gustiness and stronger winds blowing nearly parallel to the surface front.   A maximum in the wind velocity is reached just prior to the frontal passage.  As the front passes, the wind will veer and generally diminish in intensity.

Cloud Forms.     Warm fronts are nearly always well defined by typical cloud forms.  A transition from cirrus to cirrostratus to altostratus to nimbostratus is the rule.  Fractostratus and stratus and occassionally fog are observed in the immediate vicinity of the warm front.  Within the warm mass away from the front, only typical air-mass clouds are found.  Cumulu-type clouds will form in the warm air being lifted if the air mass is convectively unstable.  It is also under such conditions that mammatocumulus are observed from the ground.

Precipitation Forms.     The precipitation area of warm fronts extends about 300 miles in advance of the surface front.  Especially typical hydrometeors of warm-front precipitation are those associated with drizzle, rain, snow, and fog. 

Temperature Changes.     Temperature rises slowly with the first indications of the warm-front cloudy system. When the altostratus shield approaches and the surface inversion is destroyed, rapid increases in the surface temperature occur, often leading to the apparent development of fictitious warm front.  In precipitation and fog areas, the temperature drops several degrees.  When the front passes, a general and rapid rise in temperature is commonly observed. 

Dew-Point Temperature.     The dew-point temperature increases slowly with the approach of the warm front.  In precipitation areas, there is a marked increase in the dew point.   A further increase follows the frontal passage whenever the air in the warm sector is of maritime tropical origin.

Visibility.     In advance of the front particularly underneath the altostratus deck, the visibility is usually good, owing to the absence of a surface inversion.  In the precipitation area, the visibility decreases and becomes quite low just prior to the frontal passage.   After the warm-front passage, the visibility improves to the characteristic values of the air in the warm sector.

Ceilings.     The cloud ceilings in advance of the warm front follow the slope of the frontal surface as long as no precipitation takes place.  In the precipitation area, the ceilings are lower because of clouds forming within the cold air.   Very low ceilings prevail along the surface front, and a rapid improvement follows the frontal passage.



Pressure Distribution. 
    Cold fronts are located in well-defined pressure troughs whenever there is a marked density contrast between two air masses.   A careful analysis of the isobars will in most cases indicate the correct position of the pressure trough that contains the front.   This method of isobaric analysis is frequently the only possible means of locating fronts over ocean areas or regions of scanty surface reports.

Pressure Tendencies.     The isallobars of falling pressure in advance of the front usually form an elongated pattern approximately parallel to the front.   Cold front may indicate from weak pressure rises behind the frontal passage to sudden and strong rising-pressure tendencies following the frontal passage.   The isallobars of weak rising pressure behind the front are widespread and do not show strong isallobaric gradient. On the other hand, isallobars of strong rising pressure are often confined to a narrow and oval-shaped area behind the front.

Wind.     With the approach of the front, the wind will usually back until it is almost parallel to the front.   The wind will be highest at the time of the frontal passage.   A clockwise shift occurs with frontal passage. Very gusty winds and occasaionally a line squall occurs in the frontal zone.   After the frontal passage, the winds decrease rapidly for weak

Cloud Forms.     For Type I cold fronts, towering cumulus and cumulonimbus as well as stratocumulus and nimbostratus are observed and immediately behind the surface front.  Altostratus and cirrostratus usually trail far behind the surface front. Type II cold fronts are generally preceded by lenticular altocumulus that gradually increases and changes to altostratus.  With the approach of the front, the altostratus merges with nimbostratus and huge cumulonimbus in a cloud bank at the front. 

Precipitation Forms.     Continuous precipitation will be observed for some hours after the frontal passage with cold fronts of type I flow conditions while showers and sometimes thunderstorms activity of short duration will occur or slightly in advance of type II cold fronts.  Rapidly clearing conditions are typical after the frontal passage. 

Temperature Changes.     There is a pronounced drop in temperature with and following the frontal passage.   

Dew-Point Temperature.     The dew-point temperature will generally aid in locating fronts.  The fronts are oftentimes located by the drop in dew-point temperature that will be pronounced. 

Visibility.     The approach and passage of the front is marked by decreased visibility.  In the cold air, a slight improvement in visibility may be observed.   However, since as long as precipitation occurs, no marked improvement takes place until the precipitation ends. The front may be preceded by regions of poor visibility due to shower activity.   A pronounced improvement will almost always follow this type of cold-front passage. 

Ceilings.     The ceilings are very low at the time of frontal passage and lift only gradually after the front has passed the station.  Unlimited ceilings are usually found in the cold air, thus restricting aircraft operation in the vicinity of the front, and, far in advance of the cold front, ceilings are ample for operation of the aircraft



Mountain ranges greatly influence the speed of fronts and frontal slopes, and the frontal activity.  The orientation of the front relative to the mountain range and the size of the mountains as well as the stability of the air masses involved determine the amount of modification that develops.

The retardation of fronts in crossing mountain ranges frequently results in the development of new cyclonic waves on the front.  This effect is observed on the south coast of Greenland, along the south coast of Norway, and also along the southern section of the Aleeghenies in the United States.

Warm Fronts.     The figure represents vertical sections through a warm-front surface passing a mountain range.  As a rule, the slope of warm fronts is much less than the slope of mountain ranges.  Therefore, when a warm front approaches a mountain range, the warm-front surface reaches the mountain ridge first and traps a wedge of cold air along the mountainside.  Since the cold mass cannot escape, the warm-front surface will accordingly become stationary along the leading slopes of the mountains.  The precipitation area common to the lower portion of the warm front will then persist over the same region for a long time.   The upper portion of the warm front will pass over the mountains and descend on the lee side of the mountain range.  Precipitation and sometimes cloudiness are absent on this side owing to sinking motions.   When the warm front arrives over flat country, it regains its characteristic cloud and precipitation pattern

A warm front crossing a mountain.


The effect of mountain ranges is therefore primarily to widen the precipitation area and to increase the duration as well as the intensity of the precipitation on the windward side of the mountains and to decrease the precipitation area and intensity on the lee side.


Cold Fronts.     The figure shows successive vertical sections through a cold front crossing a mountain range.  The cold wedge is retarded on the windward side and accelerated on the lee side of the mountains.



If the air on the lee side of the mountains is colder than the advancing wedge of cold air, the cold front will merely travel across the top of the cold dome on the lee side of the mountains as an upper-air front.  This frequently happens during the winter east of the Rocky mountains and also east of the Alleghenies during the early-morning hours.  At that time, the stagnant air east of the mountains will be colder because of radiation processes, while the well-stirred air associated with the cold front will maintain higher temperatures and therefore overrun the stagnant mass of the lee side of the mountains.   Frontal activity will be at a minimum under such conditions until surface heating has become active in reversing the temperature conditions.

Thunderstorm conditions and squalls may develop when the upper-air front finally breaks through the ground.

If the air on the lee side of the mountains is warmer than the advancing wedge of cold air, the cold front will sweep the warm air away.   The frontal activity continues unhindered in this case.

The angle with which a cold front approaches and crosses a mountain range determines the increase in frontal activity on the windward side and the relative decrease on the lee side of the mountains.   The maximum change in frontal activity occurs when the front lies parallel to the mountain range.


Occluded Fronts.     Occluded fronts crossing the mountain ranges are influenced in the same way as cold fronts and warm fronts.  Cold-front-type occlusions behave like cold fronts, and warm-front-type occlusions cross mountain ranges like warm fronts.

Mountain ranges frequently tend to accelerate the occlusion process when a warm-sector cyclone approaches.  Under such circumstances, the warm front is retarded by the mountain range while the cold front moves unhindered until it reaches the mountain range and the stalled warm front.  This condition is observed often on the west coast of continents during winter.

Read more: Fronts



The Typhoon Committee, recently released a list of 140 tropical cyclone names adopted by its Members for use in the Western North Pacific Ocean and the South China Sea. The new naming system, which will lead to the standardization of the usage of names for tropical cyclones in the region, was approved during the 31st session of the Committee, held in the Philippines on 1-7 December 1998. This scheme is also expected to enhance alertness of the people to impending tropical cyclone hazard. The new method of assigning Asian names to tropical cyclones in the region will be implemented at the start of the new millenium on 1 January 2000.

Read more: Northwest Pacific Basin Names




REVISED LIST OF NAMES FOR TROPICAL CYCLONES WITHIN THE
PHILIPPINE AREA OF RESPONSIBILITY (Effective January 2017)

The first tropical cyclone of the year starts with the name beginning in letter A as in AURING under column 1 for 2017 and so on down the list as one disturbance succeeds another.  The 5th year (2021) will bring us back to column 1 of AURING.  In the event that the  number of tropical cyclones within the year exceeds 25, an auxiliary list is used, the first ten of which are listed under each column.

 

1 2 3 4

2017
2021
2025
2029

2018
2022
2026
2030

2019
2023
2027
2031

2020
2024
2028
2032

AURING

BISING

CRISING

DANTE

EMONG

FABIAN

GORIO

HUANING

ISANG

JOLINA

KIKO

LANNIE

MARING

NANDO

ODETTE

PAOLO

QUEDAN

RAMIL

SALOME

TINO

URDUJA

VINTA

WILMA

YASMIN

ZORAIDA

AGATON

BASYANG

CALOY

DOMENG

ESTER

FLORITA

GARDO

HENRY

INDAY

JOSIE

KARDING

LUIS

MAYMAY

NENENG

OMPONG

PAENG

QUEENIE

ROSITA

SAMUEL

TOMAS

USMAN

VENUS

WALDO

YAYANG

ZENY

AMANG

BETTY

CHEDENG

DODONG

EGAY

FALCON

GORING

HANNA

INENG

JENNY

KABAYAN

LIWAYWAY

MARILYN

NIMFA

ONYOK

PERLA

QUIEL

RAMON

SARAH

TISOY

URSULA

VIRING

WENG

YOYOY

ZIGZAG

AMBO

BUTCHOY

CARINA

DINDO

ENTENG

FERDIE

GENER

HELEN

IGME

JULIAN

*KRISTINE

*LEON

MARCE

*NIKA

OFEL

PEPITO

QUINTA

ROLLY

SIONY

TONYO

ULYSSES

VICKY

WARREN

YOYONG

ZOSIMO

AUXILLIARY LIST

ALAMID
BRUNO
CONCHING
DOLOR
ERNIE
FLORANTE
GERARDO
HERNAN
ISKO
JEROME
AGILA
BAGWIS
CHITO
DIEGO
ELENA
FELINO
GUNDING
HARRIET
INDANG
JESSA
ABE
BERTO
CHARO
DADO
ESTOY
FELION
GENING
HERMAN
IRMA
JAIME
ALAKDAN
BALDO
CLARA
DENCIO
ESTONG
FELIPE
GOMER
HELING
ISMAEL
JULIO

 

 Note: With * is a replaced name of the decommissioned 2016 Tropical Cyclones.

 

 

Read more: Philippine Tropical Cyclone Names




Oceans and seas have great influence on the weather of continental masses. A large portion of the solar energy reaching the sea-surface is expended in the process of evaporation. These water evaporated from the sea/ocean is carried up into the atmosphere and condenses, forming clouds from which all forms of precipitation result. Sometimes, intense cyclonic circulations occur which is what we call the tropical cyclones.

Read more: Tropical Cyclone





The information and pictures presented here were researched, compiled and simplified from varied resources about meteorology; such as books, publications and internet sources, among them:


  • CORBIS PICTURE GALLERY
  • EARTH SCIENCE, Teacher's Edition, Academe/Scott, Foresman, 1987
  • HANDBOOK OF METEOROLOGY, Bollay, E,
  • LIGHTNING and THUNDERSTORMS, Hernando County Board of Commisioners, at http://www.co.hernando.fl.us/em/thunder.htm
  • LIGHTNING and THUNDERSTORMS, Pasco County Office of Emergency Management, at http://www.pascocountyfl.net/oem/thunder.asp
  • NOAA TECHNICAL MEMORANDUM NWS SK-145
  • NOVALYNX CORPORATION, www.novalynx.com
  • SYNOPTIC METEOROLOGY, Compendium of Meteorology, WMO
  • THE CLIMATE OF THE EARTH, Lydolph, Paul E., 1985
  • UNDERSTANDING OUR ATMOSPHERIC ENVIRONMENT, W.H. Freeman and Co., Neiburger, M., Edinger, J., Bonner, W.
  • WEATHER GLOSSARY, U.S. Department of Commerce, Weather Bureau, 1946
  • WEATHER PICTURE OF THE DAY, http://www.weatherpictureoftheday.com
  • John A. Day's CLOUDMAN'S GALLERY OF CLOUDS, at http://www.cloudman.com/
  • Fabio Ciucci's Anfy 2.0 applets, at http://www.anfy.com

Read more: Acknowledgement