Here’s a rare look at a rainbow over the skies of Ethiopia. Pileus clouds are formed over the towers of cumulus clouds. The cumulus clouds puff up through convection, or the upwards motion of moist air in one section. The moisture in the air condenses and forms the towers of the cloud. Above the cumulus clouds in the troposphere, ice crystals are compressed by the wall of upward-moving air. They form the little cumulus “caps” that are pileus clouds.
These clouds only form when the convective current is strong enough and the upper atmosphere has enough water in it to form the crystals. Even then, what most people see are puffs of white cloud over the top of the cumulus clouds, not dazzling rainbows.