Weather forecaster’s job is highly technical and involves numerous steps. Collecting, transmitting, and compiling weather data of the entire world are the most essential steps in weather forecasting. For the collection of data on a global scale, there is a vast network of weather stations of various categories located on land as well as seas.
At present, according to the World Meteorological Organization (WMO), there are at least 9,525 land-based observatories, 7,424 ships sending weather data from over the seas, 644 radar stations and 2306 upper-air observing stations which are engaged in collecting weather data and transmitting the same to their respective Centers.
After the information is collected, it is transmitted to three World Meteorological Centers located in Melbourne (Australia), Moscow (former U.S.S.R), and Washington, D.C. Besides, there are at present 25 regional centers.
From these centers there is a continuous flow of 15 million weather data by telecommunication system. It may be noted that weather data are shared internationally through the World Weather Watch (WWW) system. This was inaugurated by the World Meteorological Organization in 1965.
For collecting upper-air data there are 4 Polar orbiters, 5 Geostationary Orbiters and about 525 weather-satellites. The satellite photos and radio data are used to determine the conditions at different levels of the atmosphere. However, despite such a large number of weather stations, even today, a large part of the globe is not sufficiently monitored.
Once the weather data are collected, they are immediately transmitted to the three World Meteorological Centers as mentioned above from where the compiled data are sent to the National Meteorological Centre of each member country.
At present, the membership of WMO comprises 160 nations. In India the National Meteorological Centre is located in Pune (Maharashtra). From the NMC the weather information is further disseminated to numerous regional and local meteorological centers where it is used to provide more detailed forecasts for those respective areas.
Before making an accurate forecast it is imperative that the forecaster must know the current atmospheric conditions. This stupendous work is called the weather analysis. Because the conditions in the atmosphere change rapidly, this work must be done as rapidly as possible.
In this job, help is taken from the modern most sophisticated high-speed computers. Besides the collection of enormous quantity of weather data, it must be presented in such a form that it may easily be understood by the forecaster.
This task is done by putting the weather information on a series of synoptic charts. Remember that all the weather stations, whether located on land or on ships, report the atmospheric conditions 4 times each day at 000, 0600, 1200 and 1800 hrs Greenwich Mean Time.
Then the surface weather charts are produced by first plotting the data procured from selected weather stations. By international agreement the data must be plotted by using the symbols illustrated.
Generally the plotted data include the following weather elements: temperature, dew-point, pressure and its tendency (whether falling or rising), cloudiness (height, type and amount), wind speed and direction, and both current and past weather.
It is worthwhile to note that the data are always plotted in the same position around the station model so that their reading may be made easier. However, the only exception to this rule is the wind direction which is shown by arrows oriented with the direction of the wind.
After the plotting is completed isobars and fronts are also drawn on the weather charts. Ordinarily the isobars are plotted at intervals of 4 millibars so that the weather charts do not become clumsy and congested.
The isobars are drawn by estimating their positions as accurately as possible from the pressure readings supplied by the weather stations.
In order to bring out clearly the overall picture of circulation that the charts try to depict, the irregularities of the isobars are smoothed out by the analyst. The construction of isobars is followed by plotting of centers of high and low.
As we are aware, the fronts are boundaries separating different types of air masses. The fronts, therefore, can easily be located on weather charts by noting the places where weather conditions change abruptly.
With a view to locate the fronts most accurately, the following changes are identified on surface weather charts : (a) marked temperature contrast over a short distance; (b) clockwise changes in wind direction by as much as 90 degrees; (c) humidity contrasts; and (d) clouds and precipitation patterns which give a definite clue to the position of fronts.
However, when the above mentioned contrasts are not so prominent, upper-air charts provide an invaluable aid for identifying the fronts. Most recently available weather charts are also useful guide to estimate the current position of a prevailing front.
With a view to present a complete picture of the three- dimensional atmosphere, the surface charts are supplemented by upper-air charts drawn on a regular basis for 850-, 700-, 300-, 200-, and 100-millibar surfaces. These charts show height contours instead of isobars to depict the pressure field.
A contour interval of 60 meters is commonly used. The upper air charts also contain isotherms in the form of dashed lines drawn at an interval of 5 degrees. Thus a series of upper air charts enables the forecaster to get a three-dimensional view of the atmosphere.
Now, the forecaster is in a position to evaluate the current conditions of the atmosphere. In addition, numerous other special-information charts are provided.
They include Daily Weather Map, a 500- millibar height contour chart, a highest and lowest temperatures chart, and a precipitation chart. After carefully examining these charts, a pictorial view of the weather systems as they move across any country is provided.