Timelapse footage of violent convection in a developing thunderstorm. Strong updrafts give the cloud a bubbling, cauliflower-like appearance, and the tops of the updrafts are capped by pileus clouds. These thin altostratus clouds form when the updrafts raise moist air into cooler air at higher altitudes, causing condensation. The pileus clouds are quickly caught up by the growing clouds below. Pileus clouds are a good indicator of a particularly active storm. Filmed in the USA on a summer afternoon.