Horseshoe Vortex – The Gentle Twister

When we think of twisting clouds, we tend to picture the loud ones: funnel clouds, tornadoes – the dramatic tuba features that descend from storm bases and sometimes earn the nickname ‘twisters’. But May’s Cloud of the Month is a very different sort of sky-twist. The horseshoe vortex cloud is small, short-lived and harmless. You can think of it as a tiny twister on its side.

The horseshoe vortex is not an official cloud classification but a fleeting form that can appear around small Cumulus clouds. These are the low, puffy clouds of fair-weather days, formed by rising bubbles of warm air, known as thermals. The Sun heats the ground unevenly, and some surfaces – dark fields, roads, bare rock, mountain slopes – warm more readily than others. Air above these patches becomes buoyant and rises. As it climbs, it expands and cools, and if it contains enough moisture, some of its water vapour condenses into cloud droplets.

Wayde Margetts (Member 37,625) was descending from the summit of Frenchmans Cap, in Tasmania’s Franklin-Gordon Wild Rivers National Park, Australia, when he noticed one of these little Cumulus clouds doing something unexpected. It had curled into a delicate arch, with its ends tucked over like the tips of a moustache. He had spotted the elusive horseshoe vortex cloud.

As its name suggests, this cloud forms within a vortex, a spinning flow of air, and in this case, the spin develops at the top of a rising thermal. The middle of a thermal is usually the warmest part, so it rises fastest. If the lifting column encounters brisk crosswinds overhead, it can be twisted into a horizontal spin. When the pressure inside this little whirl is low enough, and the air moist enough, condensation makes the vortex of air visible. The result is a cloud that curls over on itself, its centre rising more quickly than its ends to form the distinctive horseshoe shape.

Unlike its dramatic stormy relatives, the horseshoe vortex is no cause for alarm. It does not roar or tear across the landscape. It simply appears, curls, and vanishes again – usually in the space of just a few minutes. To spot one, look near the tops of small, developing Cumulus clouds on breezy days. But spotting one requires a quick eye and a good deal of luck, for horseshoe vortex clouds are among the rarest and most fleeting of all cloud formations.

Horseshoe vortex spotted from Frenchmans Cap, Tasmania, Australia by Wayde Margetts (Member 37,625). View this in the photo gallery.