Dale Peterson (Member 34,214) was lucky to spot two horseshoe vortices leaving their prints in a field of Stratocumulus clouds over Cherry Creek State Park, Denver, Colorado, US. Horseshoe vortex clouds form in twists of air that can develop like tiny horizontal tornadoes as columns of rising air, or thermals, encounter crosswinds overhead. Rolls of cloud can develop in the low pressure in these vortices, which are then shaped into distinctive upturned curves as the rising thermals push up their middles.

For many, horseshoes are a symbol of luck and protection. They’re given to newlyweds or nailed over a threshold. But no one can ever agree which is the right way up to hang one. Some say a horseshoe with its ends, or heels, pointing upwards keeps the luck from spilling out. For others, heels down means blessings are showered onto whoever passes below. Anyone lucky enough to spot the rare and fleeting horseshoe vortex clouds will find they always have their heels pointing down thanks to the effect of the thermals lifting their middles. So that’s the debate settled right there – well, for cloudspotters, at least.