Weather Prediction by Numerical Process
Predict the weather by solving the equations of the atmosphere, one time-step at a time.
In 1922 a Quaker mathematician worked out how to predict the weather from nothing but the laws of physics and arithmetic — and got an answer that was wildly, instructively wrong.
The idea, unpacked
The weather is not magic; it is air obeying physics. If you know the air's pressure, temperature, humidity and motion everywhere right now, the equations of physics tell you how each of those will change over the next few minutes. Do that arithmetic, step a few minutes forward, then repeat — and you have computed the future.
Richardson's leap was to take that literally. He laid a grid of cells over the map and, cell by cell, ground the equations forward by hand. No folklore, no “red sky at night” — just numbers marching the atmosphere into the future. Every weather forecast you have ever seen works this way.
Where it came from
Lewis Fry Richardson was a Quaker and a pacifist who, during the First World War, served as an ambulance driver on the Western Front. Between casualties he did his weather sums by hand — a single six-hour forecast over Germany that took him the better part of two years. The manuscript was once lost in the chaos of a retreat and turned up months later beneath a heap of coal.
When he finally finished, the answer was nonsense: his calculation said the surface pressure would leap by an amount never seen in nature. He published the failure anyway, in full, in 1922 — convinced the method was right even though this attempt was not.
Why it mattered
He was right. The forecast failed only because the starting measurements were very slightly out of balance — not because the idea was wrong, a subtlety untangled decades later. The method itself is exactly what every national weather service now runs on supercomputers. Richardson simply tried to do it, by hand, eighty years too early.
An analogy
Think of a film of the sky played forward one frame at a time. Each frame is the whole atmosphere — the pressure and wind in every square of a grid. The laws of physics are the rule that draws the next frame from the current one. Richardson drew those frames with a pencil and a slide-rule; today a computer draws millions a second. The catch: if your very first frame is even slightly off, the error can grow with every new frame — which is exactly what tripped him up.
Where it sits
The dream of forecasting from physical law was Vilhelm Bjerknes', in 1904; Richardson turned it into an actual recipe. It came true in 1950, when one of the first electronic computers, the ENIAC — guided by John von Neumann and Jule Charney — produced the first successful numerical forecast, this time filtering out the fast disturbances that had wrecked Richardson's attempt. The same lineage runs on to Edward Lorenz, whose 1963 discovery of chaos (also in this Library) showed why even a perfect version of Richardson's machine can see only a week or two ahead.
the scheme is complicated because the atmosphere is complicated.
After so much hard reasoning, may one play with a fantasy?
In this respect he is like the conductor of an orchestra in which the instruments are slide-rules and calculating machines.
But instead of waving a baton he turns a beam of rosy light upon any region that is running ahead of the rest, and a beam of blue light upon those who are behindhand.
Perhaps some day in the dim future it will be possible to advance the computations faster than the weather advances and at a cost less than the saving to mankind due to the information gained. But that is a dream.