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 5 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 can happen when it encounters a stiff cross wind passing overhead, which can set up a horizontal spin at the top of the thermal. And the moisture and temperature conditions need to be just right too, so that where the pressure drops within this horizontal tube of spinning air and so the air temperature decreases slightly a twisting ribbon of cloud can form. The thermal keeps rising from below and pushes up the centre of the horizontal vortex of cloud, distorting it into an upside upside-down curve 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