Flood, drought or scorcher, the weather is a British obsession. William Hartston examines modern theory and ancient saws
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.

Berries: Haws and heps portend cold winters (Sir Francis Bacon).

Cats 1: A cat sitting in the window washing its ears is a certain presage of rain - and the wind will blow from the way the cat faces.

Cats 2: "The sailors declare there is somebody on shore keeping a black cat under a tub, which it stands to reason must keep us in harbour" (Charles Darwin, Diary of HMS Beagle, 1831). Shutting up a cat was believed to cause strong winds.

Clergymen: A clergyman on a ship leads to bad weather.

Corns: A sign of rain.

Corpse: A corpse on board ship causes storms.

Ducks: "Ice in November to bear a duck, the rest of winter will be but muck."

Eggs: Eggs on board create contrary winds.

Ferns: The cutting or burning of ferns causes rain.

Goats: For a favourable wind, hang a he-goat to the boat's mast; especially if you have a clergyman, egg, prostitute or dead hare (q.v.) on board the ship.

Guinea fowl: The constant cry of "cognac, cognac" of these birds brings sunshine and good luck to a farm.

Haircut: "No living man has the right to cut his nails or hair on a ship; that is, unless the wind is blowing a hurricane." (Petronius, Satyricon, AD 65).

Hanging: Brings winds and roaring tempests.

Hare: A dead hare brings bad weather to a ship.

Knife: If becalmed at sea, a knife stuck in the mast will bring wind.

Marriage: Among fishing communities of Aberdeen, marriages are believed to bring bad weather. Leave weddings until the end of the herring season.

Peacock: Its "loud and harsh clamour" foretells rain.

Peony: A peony set ablaze in a rowing boat countermands rough weather (11th century).

Prostitute: A whore on board ship causes storms. The best remedy is to throw her overboard as a sacrifice to Neptune.

St Luke: 18 October, St Luke's Day, is usually warm and dry (apparently).

St Swithin: If it rains on St Swithin's Day - 15 July - it will continue raining for 40 days. In fact, an analysis published in 1894 revealed that the average number of rainy days following a wet 15 July was 181/2, while after a dry 15 July, an average 1

91/4 rainy days followed.

Thunder: Pliny says thunder heard on your left-hand side is lucky "because the east is on the left side of the heavens".

Whistling: If becalmed, whistle to raise a wind.

The greatest weather predictions of all time I am going to bring floodwaters on the earth to destroy all life under the heavens, every creature that has the breath of life in it. Everything on earth will perish.

God to Noah, personal communication, Genesis, 6, 17

A woman rang to say she heard there was a hurricane on the way. Well don't worry, there isn't.

Michael Fish, BBC TV, 15 October 1987