Welcome to the Cloud Index — a regular feature on Looking Up where we profile a type of cloud, explaining how they form, what they’re made of and how to use them to forecast the weather. You can find the full list of clouds we’ve covered in the Cloud Index index.

By far the most damaging, destructive cloud in the sky is comulonimbus. Its name, combining cumulus (Latin for ‘heap’) and nimbus (‘rain’) belies its raw destructive power. Cumulonimbus clouds produce thunder and lightning, huge downpours of rain or hail and can even spawn deadly and damaging tornadoes.

In the tropics, cumulonimbus clouds are a daily occurrence — with storms on the horizon frequently lighting up the evening skies. Nearer the poles they’re rarer, appearing seasonally during the spring and autumn as the weather starts to change.

Each cumulonimbus cloud contains as much energy as ten Hiroshima-sized atomic bombs, which goes some way towards explaining why they often form a mushroom-shape in the sky. The lower part is comprised of water droplets, while the wispy top is made of ice crystals that spread out as they reach the boundary below the stratosphere — a feature often referred to as the “anvil”.

How they form

You need three ingredients for a good cumulonimbus cloud. Heat, moisture, and instability. The heat sparks off convection, just like a cumulus cloud. As the moist air rises it cools and condenses, forming a cloud, and instability means that it can keep on going.

However, what goes up must come down — which usually results in a cumulonimbus cloud killing itself. If there’s some wind shear (a change in wind strength or direction with height), however, then the updraft can become tilted and sustain itself for much longer.

What they mean for the weather

A cumulonimbus bearing down on you isn’t good. It means torrential downpours of rain and/or hail and lightning strikes. If the storm is rotating, it can even mean tornadoes in some parts of the world. Once they dissipate, however, they tend to have a cleansing effect on the atmosphere — leaving cool and clear air behind them.

Anything else?

There are an estimated 2,000 thunderstorms going off at every moment around the world, the vast majority of which are in the tropics, with lightning striking the ground about 6,000 times per minute. That’s a lot of storms.

But it’s nothing compared to a storm raging on the surface of Jupiter. The ‘Great Red Spot’ has been recorded for between 184 and 349 years, depending on which analysis you take, and has likely existed for even longer. It’s as big as our entire planet, and takes about six Earth days to spin around in its entirety.

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