They occur when a horseshoe vortex deforms a cumulus cloud.[2] The clouds are relatively short-lived[2] and is the last stage before one dissipates.[3] Horseshoe vortex clouds are a form of "fair-weather" funnel cloud and are similar to the shear funnel type of funnel cloud.[citation needed]
As the updraft pushes flattish cumulus clouds up & a horizontal vortex develops from differential updraft speeds... As the vortex climbs, it's caught in the faster horizontal winds aloft, & the middle part of the vortex catches the faster speeds with the ends being slower.
These clouds do not occur often because all the needed conditions rarely occur together.[3]
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