Light not getting through
Why Do Storm Clouds Look Dark?
A storm cloud is dark underneath because it is thick enough to block the sunlight.
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Step 1 of 5
Clouds are white because droplets bounce all colours.
Cloud droplets are big enough to scatter every colour equally. Mixed back together, that is white.
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Step 2 of 5
Thick clouds absorb the light before it exits.
Every bounce loses a little energy. Through a kilometres-thick cloud, light bounces so many times that little reaches the underside.
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Step 3 of 5
So dark simply means full of water.
The grey is a direct measure of how much water is up there. A very dark base is a genuinely reliable warning.
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Step 4 of 5
They get tall from powerful updraughts.
Strongly rising warm air pushes a cloud upward for kilometres. Height is what turns an ordinary cloud into a storm.
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Step 5 of 5
The flat top marks where rising stops.
At the top of the lower atmosphere, air stops getting colder. The updraught cannot rise further and spreads sideways into the classic anvil.
From above, the same storm cloud is dazzling white. Darkness is only ever about which side you are on.
The short version
Storm clouds look dark because they are thick enough to absorb sunlight before it reaches their base.
Try it yourself
Hold one sheet of tissue to a lamp, then twenty. One glows, twenty look grey — thickness, not colour.
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