Timelapse footage of a pileus cloud atop a rapidly-growing cumulus congestus cloud. The pileus is the cap-like white cloud on the cumulus at upper left. A pileus is formed by strong updrafts within the cumulus cloud pushing a moist, stable layer of air upwards so that it cools below its dew point. As such, they are indicative of rapid convection and instability, and are a sign of storm development. Filmed in Colorado, USA, in summer.