One of the rarest and most fleeting of all cloud types, the horseshoe vortex forms in a patch of rotating air that’s spinning on a horizontal axis. This can develop as a thermal encounters crosswinds overhead. If the air in the vortex cools enough, a twisting line of cloud can develop, which curves up in a horseshoe shape as the thermal lifts its middle from beneath. A horseshoe vortex cloud only hangs around for a few brief minutes before it breaks up, so you have to be quick to spot one. The formation is typically distinctive enough to be unmistakable. Nevertheless, when Melanie Dashwood (Member 54,226) saw this horseshoe vortex cloud over Godstone, Surrey, England, she said ‘it seemed that the sky was raising a sceptical eyebrow’.