Weather: Cloudy views on the origins of rain

Despite having a fundamentally mistaken view of the universe, Aristotle was able to explain everything about the weather. The formation of clouds, however, gave him considerable trouble.

Aristotle was a genius. You have only to look at his views on the weather to realise just how clever he was. Saddled with the view that all matter was composed of earth, air, fire or water, he was able to develop a complete theory of weather which explains, with remarkable correctness, some complex meteorological phenomena. But he did get into difficulty over clouds.

First, let's get the universe sorted out. Everything consists of the four elements, which may originate from each other. Fire takes top place in the elemental league table, followed by air, water and earth. (Note for linguists: the word "quintessential" refers to the "fifth essence" - the intangible quality that gives any item its distinctness.)

Earth is cold, and since the sun and stars are also solid objects, they must be cold too. "As for the heat derived from the sun," says Aristotle, "we may now explain how it can be produced by the heavenly bodies which are not themselves hot. We see that motion is able to dissolve and inflame the air; indeed, moving bodies are often actually found to melt. Now the sun's motion alone is sufficient to account for the origin of terrestrial warmth and heat."

Given a little more time and a more flexible theory, he would have been well on the way to producing a theory of conservation of energy and an explanation of how kinetic energy can transform into heat. Instead Aristotle contented himself with an explanation that the sun could heat us because of its closeness and rapid motion, while the stars could not do so because they were so far away, and the moon was no use because, although close, it moved so slowly. Another reason that the sun heats us is because "the fire surrounding the air is often scattered by the motion of the heavens and driven downwards in spite of itself".

In Aristotle's model, "the moisture surrounding the earth" is made to evaporate and rise by the sun's rays. "The exhalation of water is vapour; air condensing into water is cloud. Mist is left over when a cloud condenses into water, and is therefore rather a sign of fine weather than of rain."

So he understood the difference between water vapour - which is a colourless gas - and the small water droplets that form clouds. "The moisture is always raised by the heat and descends to the earth again when it gets cold ... when the water falls in small drops it is called drizzle; when the drops are larger it is rain."

On the matter of heavy storms, however, Aristotle made a big error in disputing the views of another philosopher. Violent showers, he reasoned (incorrectly), must occur when condensation takes place quickly. That, he decided, was when a cold cloud descends quickly into warm air; "though this is the direct opposite of what Anaxagoras says. He says this happens when the cloud has risen into the cold air."

Both were right in their realisation that rain is likely when a cold front meets a warm one, but Aristotle believed that the rain then came from the cold cloud, while Anaxagoras maintained that the cooling of a warm cloud was what led to the release of water. Since we now know that warm air can hold more moisture than cold, we must side with Anaxagoras on that point.

Aristotle was wrong, too, about the cause of large raindrops as compared with small drizzledrops. It was perceptive of him to notice the difference, but it never occurred to him that most rain started life as frozen globules of ice, which melted on its way down. And he could have worked that one out: after all, he knew that motion created heat.

Tomorrow, a more modern view of cloud formation.