Reading the sky
Why Do Clouds Have Different Shapes?
A cloud's shape tells you how the air beneath it is moving.
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Step 1 of 5
Every cloud starts the same way.
Air rises, cools, and its invisible vapour turns into droplets. The water is identical in every cloud — only the way the air rose is different.
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Step 2 of 5
Bubbles of rising air make heaps.
When the ground heats unevenly, warm patches float up as separate bubbles. Each bubble makes its own cauliflower lump with a flat base where cooling began.
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Step 3 of 5
A whole slab lifting makes flat sheets.
When a huge layer of air is pushed gently upward — over a warm front, say — it cools all at once and turns into a featureless grey sheet.
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Step 4 of 5
Height decides whether it is water or ice.
Low clouds are droplets. High ones sit where the air is far below freezing, so they are made of ice crystals — which is why they look thin and feathery.
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Step 5 of 5
A tall heap means trouble.
If the rising bubble keeps going, the heap grows into a tower kilometres deep. That is the cloud that makes hail, thunder and downpours.
A big storm tower can hold a million tonnes of water and reach higher than an airliner cruises.
The short version
Cloud shape is a picture of moving air: bubbles make heaps, gentle slabs make sheets, and height decides water or ice.
Try it yourself
Watch one cloud for five minutes. If its edges boil and change, air is rising fast underneath it.
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