Coded versions of observations of weather taken at the surface and at various levels of the atmosphere at hundreds of observing stations throughout Australia and adjacent territories, will be fed into the computer. Under program control the computer will 'recognise' the messages and subject them to validation—to make sure the message has the correct number of groups, that the station or place names are correct and that the meteorological information is consistent. For example, because of an error in transmission it may be that the present weather at an observing station is shown as 'dense fog' while the visibility is shown as 30 miles. The two statements are obviously incompatible and a program will be stored permanently in the computer to ensure that inconsistencies are brought to the notice of the duty meteorologist.

The 'recognised' correct message will then pass through the computer to form the basis for portraying the current state of the atmosphere, i.e. the production of the analysis chart. At this stage further checks will be applied by comparing observations from adjacent stations and also by comparing observations taken at three-hourly intervals from the one station.

Predetermined standards of consistency will have been built into the computer program and any observations which vary from these standards will appear on the printer so that the duty meteorologist may investigate them and decide whether they should be used in their original form.

The analysed charts consisting of contours of pressure and other meteorological elements will be drawn directly under computer control. These charts will appear on a map pre-printed with the outline of the Australian coastline. At the same time the computer will be preparing the prognostic charts—a set of charts depicting the future state of the atmosphere. These charts form the basic tool for the meteorologist to prepare his forecast. It is this process of producing prognostic charts which sets the need for very large computers with a high speed of operation.

Because of the repetitive nature of the tasks in which the prognostic charts are built up from a large number of evenly spaced grid points, the relevant calculations must be performed for each grid point separately. The number of calculations also varies directly in relation to the duration of the forecast period: thus a 24 hour forecast will generally need twice as many calculations as a 12-hour forecast.

Bureau of Meteorology