How do clouds hold their shape for so long?
In: 12
They don’t. They’re just really really really huge (hundreds of metric tons of water, spread so thinly that it’s not dense enough to fall), so the changes seem small and gradual by comparison to the size that they are.
[Here’s](https://www.youtube.com/watch?v=Qu7mcKZgqv0) a good time lapse where you can see that while the edges of clouds are active, as a whole they seem to keep a “shape”.
This is because clouds are the visible result of things you can’t see:
– The amount of water vapor in the air
– Air currents and their vertical motion
– The temperature of the air at various levels in the atmosphere
Puffy clouds form when the sun heats up the ground, causing warm, humid air to rise until the water condenses. Voila, a cloud.
But that air that just rose cools off at altitude, and wants to go back down again. So all around the cloud, dry air is descending to the surface. Those form the spaces between clouds.
What you see in a time lapse is this system finding equilibrium: updrafts constantly sucking in warm, humid air from the surface, at a fairly constant rate to keep the cloud’s shape.
When the sun stops providing energy to the ground, there isn’t enough moisture to sustain clouds anymore, and so begins a clear evening.
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