Here's a rare look at a rainbow over the skies of Ethiopia. Find out how this cloud — known as a "pileus cloud" — brightens the atmosphere.
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.
As with any rainbow effect, the light has to be precisely placed. Light has to hit the cloud and diffract through the prism of the drops. But when light hits the cloud from behind, it usually hits the eye of the beholder head-on as well. This means that any rainbow effect is drowned in a flood of white light. Iridescent clouds are only visible when something blocks out the light from the sun while still showing the light through the clouds. In this case, the dark cumulus cloud blocks out the sunlight, while the pileus cloud shimmers.