A SIMPLE weather forecast can be made by using what we might call Scroop's Axiom after Sir Stephen Scroop in Richard II who was the first to put it tastefully in words.
Men judge by the complexion of the sky,
The state and inclination of the day.
It may be evident, for example, that it is becoming cloudier and seems like rain or there may be breaks in the cloud and one might reasonably draw the conclusion that the sun may shortly shine.
Those who wish to forecast a little further ahead, on the other hand, might use the Richmond Rule, enunciated by the Earl of Richmond in Richard III
The weary sun hath made a golden set,
And, by the bright track of his fiery car,
Gives token of a goodly day tomorrow.
The disadvantage of these "do it yourself" forecasting methods, however, is that the conclusions are arrived at in almost complete ignorance of events beyond the horizon. Ruskin's Reminder, offered by John Ruskin, the 19th century art critic and amateur meteorologist, describes their limitations thus "The meteorologist is impotent if alone his observations are useless, for they are made upon a point, while the speculations to be derived from them must be on space".
To prepare a forecast for any, useful length of time ahead, it is desirable to have information about the current state of the atmosphere over as wide an area as possible. The minimum requirement is dictated by the range of the forecast the further ahead the prediction, the greater the area from which observations are essential. To obtain reliable predictions for only a few hours ahead, for example, it may be sufficient to know what is happening in the atmosphere within a radius of 100 miles or so, since any weather systems outside this distance are unlikely to arrive within the time scale.
A 24 hour forecasts, on the other hand, requires the net to be cast considerably wider in the case of Ireland, it is necessary to assess the three dimensional structure of the atmosphere over the whole of the north Atlantic and much of Europe.
As the time scale approaches a week or so, the extending ripples from minor meteorological features in the southern hemisphere may well dictate the future weather sequence many thousands of miles away, so it is necessary to have observations covering the whole globe. As the Lorenz Conjecture formulated by Edward Lorenz, father of chaos theory rather nicely has it "Could the flap of a butterfly's wings over Brazil spawn the next tornado up in Texas?"