Mammatocumulus are impressive clouds. They are described as eerie, even foreboding. When you see them you get the feeling that something powerful is behind them, that something is going to happen. It’s true. Something powerful is there.
Mammatocumulus are associated with cumulonimbus, the most powerful of clouds. These are the thunderclouds, the ones that go boiling up, growing to great heights, getting so thick that sunlight can’t get down to their black bases. When they get so tall that they can’t grow any more, the wind up there blows their tops sideways, into the anvil characteristic of their cloud type. Once clear of the column of rising air, the saturated air in the anvil sinks. When there is enough moisture in this cold, sinking air to outlast the heating that results, clouds form in the lower atmosphere. pushing down into those ominous bulges. Mammatocumulus is a rare cloud that forms in sinking air rather than rising air. What gives mammatocumulus those well defined pouches is the air rising back up in the interstices between them.
There is definitely precipitation assiciated with mammatocumulus, though it comes not from them so much as from the cumulonimbus accompanying them. Along with the lightning and thunder and downdrafts and other severe weather. And finally, yes, mammatocumulus are so named because of their appearance.