The focus of this column yesterday was cumulus clouds. When the weather is calm and warm, and the sky relatively clear, surface heating by sunshine produces ascending columns of air - here, there, and sometimes nearly everywhere, around the countryside.
If the atmosphere is relatively humid, a cloud forms near the top of each of these invisible pillars - a bulbous rounded cloud with a flat base, resembling a clump of cotton-wool or a giant cauliflower.
Literary experts are unanimous in the view that when William Wordsworth "wandered lonely as a cloud that floats on high o'er vales and hills", it was fair-weather cumulus he had in mind.
If you begin to contemplate a cumulus cloud, however, you might begin to wonder how it floats at all. It is composed of a myriad of very fine water droplets whose diameters range from a ten-thousandth to a hundredth of a centimetre, each one so small that it would take 200 million of then to provide a single teaspoonful of water.
But if you add together the weight of all the individual little drops, even a small cumulus cloud may hold anything from 100 to 1,000 tonnes of water. And water, after all, is about 800 times heavier than air.
The whole celestial ensemble ought to respect the law of gravity, and collapse into a soggy pool around our feet.
Happily this does not happen. According to a refinement of the law of gravity, Newton's Second Law of Motion, a falling object, left to its own devices, should accelerate and fall faster and faster as the seconds tick. But the same object, falling through the atmosphere, is also subject to resistance from the air, and the greater its rate of descent, the greater this resistance force, or "drag".
In due course a point is reached where the tendency for the rate of fall to increase is balanced by the action of the air slowing the object down; it reaches what is called its terminal velocity - a constant rate of descent.
Light objects tend to have a rather low terminal velocity - as can be seen in the case of dandelion seed. Cloud droplets have a very low terminal velocity indeed - of the order of only a centimetre or two per second.
This rate of movement is negligible compared with the other motions in the atmosphere - such as the swirls and eddies caused by wind. So unless the drops grow big enough and heavy enough to fall as rain, the cloud stays suspended indefinitely in the sky above our heads.