Numerical weather prediction is based on a mathematical model of the atmosphere. Given values of pressure, temperature and humidity at a certain spot, it is possible by computer to calculate the expected values of these elements in the same place at some time in the future. This operation, carried out for hundreds of points on a weather chart, allows a new weather map for several hours ahead, to be arrived at.
This concept was developed early in the last century by Lewis Fry Richardson. Richardson devised a series of equations that encapsulated many of the known principles of physics, and his forecasting technique consisted of applying these equations to advance the forecast in short time-steps. An intimidating amount of calculation was involved, and in those pre-computer days there seemed no practical way it could be implemented.
Richardson made a whimsical suggestion: "Imagine," he wrote "a large hall like a theatre, except that the circles and galleries go right through the space usually occupied by the stage. The walls of this chamber are painted to form a map of the globe, and a myriad of persons are at work upon the weather on the part of the map where each sits." The work, he reckoned, would be supervised by one who would be "like the conductor of an orchestra in which the instruments are slide-rules and calculating machines." He called his dream "the weather factory".
But with the arrival of the electronic computer Richardson's ideas became a real possibility, and refined over the decades, they now form the basis of modern weather forecasting. Powerful computers are loaded with the latest set of weather observations; they crunch the numbers for an hour or two, and then produce a chart that show the likely weather pattern for some future time. The predictions have increased steadily in accuracy as the models have improved with time, but suffer from the disadvantage that the forecaster receives a new chart from the computer only every three hours, or maybe even only every six hours.
To over come this difficulty, meteorologists are now working towards a system whereby the computer would continuously ingest new weather observations and satellite data as they became available, while at the other end the machine would make available continuously updated weather charts, based on the very latest information. The working title given to this ambitious system is the "continuous weather engine" - the ultimate 21st century realisation, perhaps, of the "weather factory" that Lewis Fry Richardson imagined all those years ago.