Also known as the « small sheep ». The name comes from the Latin words cumulus (heap) and humilis (small, close to the ground) and describes an accumulation of small clouds forming at low altitude, from a few hundred metres up to 3000. It is the smallest of its cloud family, the “cumuliforms” clouds, which means they develop vertically. Its cousins are, for example, the cumulus congestus and cumulonimbus.
Often referred to as “fair weather cloud”, it comes along a great blue sky ans is easily recognisable by its fluffy looks. Grey-ish when seen from the ground, it shines bright white from above. Develops in group, a real flock of sheep in the sky.
The sun heats up the air close to the surface. The warmed air becomes less dense and rises in altitude like an air balloon. Up-there, the pressure decreases and the air gets colder, condensing water vapour into small droplets of water, just as you create a cloud when blowing into a cold air. The small fluffy cloud that appears then is a cumulus humilis, a white hat above a rising air flow. Why white? That is not water vapour, as it is invisible to the human eye. It is the tiny droplets of water, acting as millions of mirrors, that reflect the light from the sun.
Lacking time to travel, cumulus humilis cannot go far. The dry surrounding air evaporates the droplets of water and nibbles at its sides. It only takes around fifteen minutes to eat it all up. Yet sometimes, if the air close to the surface is really warm and moist, clouds can grow higher and wider (see congestus).
When to see it
Cumulus are fair weather clouds. In the tropics under trade winds, we can see them all year long, almost every day.
Did you know?
The name cumulus (or stratus and cirrus for other kinds of clouds) was given by a British meteorologist in the early 19th century, Luke Howard. At the same time, a French scientist named Lamarck made a different proposition: “veiled”, “bloated” or even “imp clouds”. As poetic as it was, the more rigorous British version was kept as the reference.