Here ’s a rare tone at a rainbow over the sky of Ethiopia . retrieve out how this swarm — have a go at it as a “ pileus cloud ” — brightens the atmosphere .
Pileus clouds are formed over the tower of cumulus clouds . The cumulus clouds pant up through convection , or the upwards motion of moist air in one subdivision . The moisture in the aura condenses and forms the towers of the swarm . Above the mound cloud in the troposphere , ice crystals are compressed by the wall of upward - moving air . They form the small cumulus cloud “ cap ” that are pileus clouds .
These cloud only form when the convective flow is strong enough and the upper atmosphere has enough water in it to form the crystal . Even then , what most hoi polloi see are puffs of clean cloud over the top of the cumulus clouds , not dazzling rainbow .
As with any rainbow effect , the light has to be exactly placed . Light has to pip the swarm and diffract through the prism of the drops . But when light hits the cloud from behind , it normally hit the optic of the beholder brain - on as well . This think of that any rainbow effect is drowned in a flood of blank light . Iridescent clouds are only visible when something blocks out the light from the Sunday while still testify the light through the clouds . In this case , the dark cumulus swarm blocks out the sun , while the pileus cloud shimmers .
ViaNew Scientist . Image : Esther Havens .
PhysicsRainbows
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