THE FACTS The ancient world believed in a river flowing between the Earth and the heavens, from which rain fell at the whim of the gods. Though explaining sunny intervals and scattered showers, this is no longer considered a useful model for weather prediction.
The modern view is that climate depends on four factors: an area's latitude; its relative distribution of land and sea; its height above sea level and general topography; and its location in relation to ocean currents.
Both wind and rain are ultimately caused by the sun, and the Earth's motion about it. At the Equator, the sun is never far from being directly overhead and its heat is concentrated. Nearer the poles, its rays hit the Earth obliquely and the heat is more dissipated. These unequal temperatures cause the air currents we know as wind. The Earth's rotation deflects these winds, making them blow from the north-east in the northern hemisphere and the southeast in the southern.
Rain, hail, sleet and snow all begin with the condensation of water vapour about small particles called cloud condensation nuclei. As the air rises and cools, these particles coalesce into droplets large enough to fall to the ground. The simple-minded v i ew is that cold air holds less water vapour than hot, but to sound impressive one must talk about the adiabatic cooling by expansion of rising air currents and saturation vapour pressure increasing exponentially with decreasing temperature.
Accurate prediction of the weather depends on good measurements of present atmospheric conditions and solving the complex mathematical equations that predict how the various air currents will move and interact in the near future.
In recent years, however, mathematics has given weather forecasters the perfect excuse for getting things wrong. Chaos theory proves that infinitesimally small changes in original conditions can lead to vast differences in predicted outcome. In the clas s ic example, a butterfly fluttering its wing in Tokyo can make the difference between sunshine and a tornado in New York a month later.
So what has been fluttering its wings in Wales this week? The storms and resulting floods are due to a combination of factors: an intense depression causing the air to spiral upwards rapidly into clouds, the push of high ground encouraging the formation of rain, high pressure over the Continent leading to mild and humid weather, warm and wet south-westerly winds from the Atlantic. Even global warming may be a contributory factor.
Or the excessive rain could, as Aristotle believed, be connected with the large number of kine receiving bulls and conceiving this summer.
THE FANTASIES Ash tree: To turn a tempest aside, fix four staves of an ash tree into a cross and make a cross sign with it (16th century).
Beetles: Treading on a beetle is a certain portent of rain. …