From the Latin cumulus (heap) and nimbus (raincloud), it is a dense cloud, wide from 5 to 15 kilometres and rising up to 15 kilometres. Nicknamed Storm cloud, that’s the one we are looking for. Time to put on your raincoat and go storm chasing!
Looks like a big mushroom, with a centre column topped by a wide hat. Also nicknamed anvil for this characteristic shape. The roof of the anvil, crushed against the top of the troposphere, is composed of ice crystals that give it a smooth or stringy look. White from afar, dark when seen from under.
Like his cousins, the cumulonimbus grows when the air near the surface is warm. Less dense, it rises, then cools in altitude and its water vapour condenses to create droplets. If in addition the air is very moist, many droplets form. When vapour condenses, it warms the atmosphere around it, just as it feels warm when water vapour condenses on your nose when drinking a hot chocolate. When many droplets form, the heat produced allows the air at the heart of the cloud to remain lighter than the air surrounding it, and rises even higher, way beyond the altitude of a cumulus. Nothing can stop it: the higher it goes, the more its vapour condenses and the more heat helps it get higher. It rises as long as the air inside the cumulonimbus is warmer than the air around. The air only stops when it has condensed all or almost all its water vapour, or when it meets warmer air, as is the case when it reaches the top of the troposphere, located between 15 and 20 kilometres in the tropics, and between 10 to 12 kilometres in temperate latitudes. At this altitude, unable to climb any further, the cloud spreads out horizontally to form an anvil. The cumulonimbus is also a rainfall machine. Thanks to its immense vertical development, it contains huge amounts of water that comes pouring down in rainfall lasting from 10 to 30 minutes.
A cumulonimbus is the building block of a storm. Some remain isolated and die out in about 30 minutes, giving way to the short rainfall we found in summer. And some assemble to form larger storms (see squall line, supercell, cyclones).
When to see it
Common in the summer, when the air feels heavy (warm and moist) or in the spring with the blasts of March/April showers.
Did you know?
A cumulonimbus is extremely heavy! With the monstruous amount of water it holds, it can weigh from 50 000 to a million tons.