Cumulonimbus

How to recognise it

• An enormous tower, far taller than it is wide, often visible in full from many kilometres away.
• A flat, spreading anvil at the top (incus), blown downwind where the cloud hits the stable stratosphere and can rise no further.
• A fibrous, soft-edged upper section — the ice crystals up there have lost the crisp cauliflower outline of cumulus.
• A dark, often ragged base with visible rain shafts, and sometimes a lowered wall of cloud beneath it.

What weather it signals

• Heavy showers, thunder and lightning. Cumulonimbus is the only cloud genus that produces lightning at all.
• Hail, sudden violent wind gusts and downbursts are possible, and the most severe examples produce tornadoes.
• The weather is intense but short-lived in any one place — a single storm cell usually passes within 20 to 60 minutes.

How to photograph it

• Photograph it from a distance. Under the cloud you see only grey; the anvil and tower structure is only readable from several kilometres away.
• Use a wide angle and include a low horizon — the point of the picture is the sheer vertical extent.
• Late-day sun striking the tower from the side gives the most dramatic contrast against a dark sky.
• Do not stay outdoors to get the shot. If you can hear thunder, you are within range of a strike.

Fun facts

• A large cumulonimbus can release energy comparable to a substantial nuclear weapon over its lifetime, almost all of it as latent heat from condensing water.
• The flat anvil forms because the tower hits the tropopause — a stable layer the rising air cannot penetrate — and is forced to spread sideways instead.
• The strongest storms punch a small dome above the anvil called an overshooting top, a sign of a violently strong updraught.

Easily confused with

Frequently asked questions