VI.

Predictability and Chaos

We have a restless atmosphere. Theory, experiment, and experience suggest there can be no atmospheric state in which the major wind systems remain fixed in position. This is fortunate, because if depressions always took the same track then areas affected would be waterlogged, with deserts elsewhere.

A perfect model, complete knowledge of the initial state of the atmosphere, and error-free computation would still not enable us to forecast accurately for more than a week or two ahead. There is overwhelming evidence that the atmosphere is inherently unstable to small-scale features. For example, an individual thunderstorm might affect the subsequent development and track of a large depression. The thunderstorm itself could result from a brief sunny spell. This means that, whereas the largest weather systems may be predictable for seven days or so ahead, there is no prospect of detailed forecasts beyond this.

The circulation of the atmosphere may be regarded as a combination of two different types of system. A roulette wheel is essentially random because minute differences in the ball's progress, far smaller than can be measured, have an overwhelming influence on the result. By contrast a ball released in a smooth rotating saucer will oscillate in a predictable manner: it is said to be deterministic. The atmosphere is partly deterministic and partly random—a so-called chaotic system. Oscillations in the strong jet streams high in the atmosphere may be similar for many days, and weather systems develop, move, and decay in a regular manner. Suddenly the pattern of upper winds subtly changes, and the tracks of depressions become quite different, or they become stationary. Areas that had been subjected to days of wind and rain with little respite then lie under the clear skies of an anticyclone.