Flat Cloud Bases: Where the Air Turns to Water

A fair-weather cumulus is allowed to do what it likes above and almost nothing below. The tops boil and lean and change shape while you watch; the bases sit on one level, as if the sky had a floor at four thousand feet and the clouds were resting on it. Every cloud in view shares that floor, which is the odd part — a hundred separate parcels of air, none of them in contact, agreeing on a height to within a few metres.

The reason is that they are all measuring the same thing. Air warmed at the ground rises and cools by about one degree for every hundred metres of climb, and it keeps its moisture the whole way up. At the altitude where its temperature meets its dew point, the vapour condenses and becomes visible; below that line there is just as much water in the air, only still in gas. Meteorologists call it the lifting condensation level, and you can estimate it from the ground: take the spread between temperature and dew point in degrees Celsius and reckon roughly a hundred and twenty metres for each one.

For a photograph the flat base does something a boiling top cannot: it gives the frame a horizontal line above the horizon, and the eye reads it as a ceiling rather than as an object. The undersides are grey because the light has been scattered out of them on the way through, so the picture divides into bright tops, flat shadowed bottoms, and the blue gap between. It is a composition the weather builds on its own, and it holds up on a screen because the line is straight enough to sit under a row of icons without arguing with them.