The Golden Horseshoe
One of the rarest cloud formations, and quite possibly the most difficult one to spot, is the ‘horseshoe vortex cloud’. This example, gilded by the rays of the setting sun, was spotted by Mark Evans (Member 8,452) over Cwmdare, in the Cynon Valley, South Wales, UK. Mark tells us it only lasted some 15 minutes before the distinctive curve of cloud dissipated away.
Conditions need to be just right for this short-lived cloud to form. A rising air current, often a thermal lifting off the sun-warmed ground, needs to take on a twisting motion as it ascends. This air current also needs to encounter a crosswind overhead and a ‘temperature inversion’, which when the way that the air temperature changes with altitude acts as an invisible lid on the rising thermal. And the moisture and temperature conditions need to be just right too, so that a ribbon of cloud can form as the temperature drops within the spinning vortex of the rising air. Upon reaching the invisible ceiling, this ribbon of cloud tumbles back downwards. The result is an upside-down curve of twisting cloud that resembles a horseshoe before it disappears.
Many congratulations to Mark for the May Cloud of the Month. Not only did he manage to capture a horseshoe vortex cloud, he got a golden one!
Photograph © Mark Evans