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.