About Horseshoe vortex

What a subtle little wisp of cloud the horseshoe vortex is! It is easily missed by anyone other than the most keen-eyed cloudspotter, intent on adding it to their collection. The rare and fleeting horseshoe vortex cloud appears for just a few minutes before breaking up or evaporating away. Anyone lucky enough to spot one must take a photo if they want to be believed by their cloud-collecting friends.

This cloud forms in a region of rotating air, or vortex. While the familiar orientation for a vortex is vertical (see tuba), they can occasionally develop on a horizontal axis. This is when the gently rotating crescent of the horseshoe vortex cloud can form. The movement of air seems to result from a thermal updraught that is sent into a spin as it reaches a horizontal crosswinds overhead. Rarely are conditions right for a cloud to appear within the vortex but, when they are, the rotating air cools enough to develop a twisting crescent of cloud. It starts out mostly horizontal, but it is soon distorted into a curve as the thermal below continues to push upwards. It lifts the middle of the rotating tube of cloud more than the sides and – hey presto! – the horseshoe shape is formed.

This beautiful little cloud may not lead to any precipitation, but it will rain down luck upon anyone fortunate enough to spot it.

Image: Spotted over Salo Corner, Saint Louis County, Minnesota, United States by Suzanne Winckler.