Tuesday, October 09, 2007

Whither, and from whence, the weather?

The weather today was grim; in fact, it was grimmer than the grimmest spell in a grimoire written by the Brothers Grimm.

The jet-stream moved Southwards this year, and as a consequence, the UK's Summer weather was dominated by a succession of North Atlantic depressions. If the jet-stream remains so over the next few months, a particularly cold Winter is predicted.

It is appropriate, then, to recall Lewis Fry Richardson, the man who invented the modern weather forecast as a numerical solution of a coupled set of differential equations. Fry's astonishing efforts are recalled in Peter Lynch's recent book, The Emergence of Numerical Weather Prediction: Richardson's Dream. Reviewed in the current issue of American Scientist, Lynch points out that:

[Fry] set out to calculate the weather. He built a working mathematical model of the Earth's atmosphere, based on straightforward physical rules. For example, one rule says that if regions differ in barometric pressure, then air will start to flow along the gradient toward the lower-pressure area. Richardson filled in initial values of pressure, wind velocity and so on, and then traced the model's evolution over time.

Models based on essentially the same principles now run on supercomputers capable of trillions of operations per second. Richardson, however, worked entirely with pencil and paper, using a sheaf of forms he had printed up to guide the computations; his only aids to calculation were a slide rule and a table of logarithms. Furthermore, the circumstances in which he did all this arithmetic have made the project legendary. The time was World War I. Richardson, a pacifist, was serving as a volunteer with the Friends Ambulance Unit in the north of France. He performed his calculations between calls to carry wounded from the front. "My office," he reported, "was a heap of hay in a cold rest billet."

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