The sides of the common cloud are almost flat. Because of this, due to the hot air, the water vapor is prepared for rain in this cloud at a certain height at a certain temperature to be concentrated in these clouds. Sometimes when these clouds become pockets of huge waters, they are called mammatus clouds. These clouds originate when some nearby cyclones are created.
This image was taken from the city of Hastings in June 2004.
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From @emamuddin