A Whole lot of Convection Going on.
A pileus formation is a smooth cap of cloud that appears on top of a large Cumulus congestus that is building rapidly upwards in the sky. The name pileus from the Latin for a felt cap that was worn in ancient Rome. For one to form, the convection cloud below needs to be growing vigorously, like the one shown here over Kolkata, India. Illuminated by the sunlight at the centre of the image, the pileus would have been created by an updraft within the large Cumulus with speeds of around 20–50 km/h. An updraft like this can, if conditions are right, lift a layer of moist air above, causing it to cool enough to form into a smooth cap of cloud. This formation is therefore often an indicator of heavy showers to come, since it tends to form atop Cumulus that are in the process of building into the mighty storm clouds, Cumulonimbus. Usually, as the cloud continues its ascent, it grows up through the pileus cap, which is left hanging around the convection cloud’s shoulders or midriff. At this point, it becomes know as a velum, from the Latin for an awning. Many thanks to Abhishek Saigal for sending in this great Cloud of the Month.
Photograph © Abhishek Saigal.