A horseshoe vortex cloud forms in a region of air that’s rotating on a horizontal axis and then gets bent into a curve. The horizontal vortex can form when a column of air rising from the ground, known as a thermal, encounters strong crosswinds overhead, which sets its top spinning. A rotating ribbon of cloud can form where the air cools in the low pressure of the vortex. And as the thermal keeps rising, it pushes up the middle of the twisting cloud to form a horseshoe shape. The distinctive cloud feature only lasts for three or four minutes before breaking up. This makes it a hard one to spot. Seeing two horseshoe vortex clouds in the sky at once is rare, which makes app user ‘Tomkey’, who spotted this pair over Boterhoek, Flanders, Belgium, one lucky cloudspotter.