Pileus looks rather like a smooth, cloud beret, or, perhaps, a cloud version of a Donald-Trump, comb-over hairstyle. It is the most short-lived of all the accessory clouds, and also the most beautiful. It shares much in common with lenticularis and cap clouds, which form when a stable airstream rises to pass over raised ground. In the case of pileus, however, the obstacle is not rocky terrain, but something altogether more ephemeral – another cloud.

A pileus cloud is horizontal cap that appears momentarily on top of a large cloud that is developing upwards from below. As in this example of a multi-layered pileus, the cloud that it forms over can be a Cumulus congestus or it might be a young Cumulonimbus cloud. When one of these large convection clouds encounters a moist, stable airstream blowing across the top of it as it builds higher and higher in the sky, the growing cloud can cause the airstream to be lifted upwards. As it is lifted, the airstream can cool just enough for some of its moisture to condense into droplets. These evaporate as the airflow sinks back down again beyond the convection cloud.

A pileus earns high points for the cloud collector because, unlike its relation, velum, it never hangs around for long. Cloudspotters have to be sharp-eyed to add one to their collection. The vigorous convection cloud that created it inevitably continues to rise, pushing its bald head through the hairstyle. Donald Trump’s head will eventually do the same.