What is a weather forecast?
A weather forecast is 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 users of forecast are rainfall, wind and cloudiness.
How is weather forecast made?
In forecasting the weather, a Meteorologist must have an idea about the existing weather condition over a large area before he can make a reliable forecast. The forecast is based on various forecasting tools. The basic tool of a weather forecaster is the WEATHER MAP. There are two types of the basic weather map namely, the surface map and the upper-air maps. The surface map depicts the distribution patterns of atmospheric pressure, wind, temperature and humidity, while the upper-air maps show the distribution of wind direction/speed, and temperature. There are five standard levels of the upper-air maps that are observed, plotted, and prepared twice daily at twelve-hourly interval. The surface maps are made four times daily at six-hourly intervals.
1st Step: Observation
Observation of different weather elements are made simultaneously as follows:
• Surface observations of different weather parameters such as pressure, wind direction and speed, temperature of air, humidity, clouds, precipitation and visibility are made at least every three hours over land and sea using instruments such as barometer, wind vane, anemometer, thermometer, psychrometer or hydrometer and raingauge. Coastal weather stations, weather ships and ocean data buoy observe state of the sea.
• Upper air stations 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 radio sounds which records 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 thei routes at specified times.
• 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 the numerical weather prediction.
2nd step: Collection and Transmission of Weather Data
Weather observations which are condensed into coded figures, symbols and numerals are transmitted via radiophone, teletype, facsimile machine or telephone and SMS (short messaging system) 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
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 seas 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
1. 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 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.
• 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.
2. Compare the current weather maps with the previous 24-72 our weather maps level by level to determine the development and movement of weather systems that may affect the forecast area.
3. Examine the latest weather satellite picture, noting the clods formations in relation to that weather system on the current weather maps.
4. 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.
5. Examine the latest computer outputs of numerical weather prediction models noting the 24-hor, 48-hour and 72-hour objective forecast of various weather systems that may affect the forecast area.
6. Analyze the latest radar reports and other forecasting tools.
5th Step: Formulation of the Forecast
After all available meteorological information/data has been analyzed, forecasts formulations follows. This include the 24 hours weather forecast and five days extended weather outlook for selected cities.
How is the weather forecast disseminated?
After completion, the weather forecast is immediately transmitted to the PAGASA network in National Capital Region(NCR) and field stations for nationwide distribution to the tri-media, NDCC, and other concerned government agencies and clients. The automated telephone and telefax and internet website weather information systems provide the general public with direct access to the daily weather forecast.
Public Information & International Affairs Staff (PIIAS)
Science Garden, Agham Road,
Diliman, Quezon City
Telefax No. 927-9308/434-2696
Revised January 2008
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