The bottom of normal clouds is flat because the hot and humid air that rises and cools also condenses in the form of water droplets at a very specific temperature, which also usually happens at a very specific height. After the water droplets form, that air transforms into an opaque cloud.
However, under certain conditions the air pockets of the cloud generate larger drops of water or ice, which fall on the clear air and evaporate. These bags can occur in the turbulent air near a storm, and can be seen on the top of the cumulonimbus, for example.
The "mammatus" clouds that occur are especially spectacular if the Sun illuminates them laterally.
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