Producing a weather forecast

There are four steps in producing a weather forecast. First, relevant features of the atmosphere must be observed as frequently, accurately and over as wide an area as possible. Then these observations must be assembled as quickly as possible at a central point. Next they must be processed and analysed to provide a comprehensive understanding of the current weather, including the preparation of maps. Finally, forecasters must predict how the maps, and consequently the weather, will change.

The easiest observations are those at ground level. The Bureau's 60 staffed stations, 500 co-operative observers and 6000 volunteer rainfall observers gather the necessary data.

Conditions in the upper air, which have an enormous impact on the weather, are measured by radiosondes, balloon-borne instruments transmitting readings of pressure, temperature and humidity. Rising to about 30,000 metres, the balloons can also be tracked by theodolites or radar to measure wind speed and direction. Over the oceans, the considerable gaps in these upper air networks are filled by data from weather satellites.

These vast amounts of data are transmitted to the forecasting office almost instantaneously by facsimile and computer links. Supercomputers making millions of calculations a second automatically plot data on charts and also generate forecast maps. The forecaster selects and considers the information relevant to a particular forecast, which is despatched automatically to newspapers, radio and TV stations, emergency services, and other users.