Weather forecasts are provided on a variety of time-sales. Several local violent atmospheric disturbances, such as hurricanes or typhoons, hail-storms, tornadoes or other destructive gusts, are predicted several hours in advance.
Warnings by radio, televisions etc are generally issued 6 to 12 hours before the actual arrival of the storm. On the basis of the period for which forecasts are valid, the weather forecasts are classified as under:-
(1) Short-range forecasts
(2) Medium-range forecasts
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(3) Long-range forecasts.
The short-range forecasts range from a few hours to about 48 hours or sometimes 72 hours. When forecasts cover a period ranging from 3 days to about 3 weeks, they are referred to as medium-range forecasts.
In the United States of America and some other countries weather predictions for periods covering a week or a month are made as a public service and these are called medium-range forecasts.
The term long-term forecasts have been reserved for ‘general and broad outlooks’ covering a longer period of time, say, one or more seasons or years.
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At present the techniques used for making long-range forecasts are still in an experimental stage. It is yet to be seen whether the techniques being applied to such forecasts give the desired results.