Timelapse footage of fast-growing cumulus clouds with pileus clouds at their tops. The pileus cloud appears as a horizontal cap, and is most visible at centre right here. They form as the rising air in the cumulus cloud pushes a parcel of moist air upwards, causing it to condense. They are indicative of powerful updrafts, and therefore a good sign of an impending storm. Filmed in Colorado, USA, in summer.