Towering cumulus clouds. Time-lapse footage of cumulus congestus clouds illuminated by the late afternoon Sun. These clouds show modest growth and potential for further development. A cumulus cloud can grow upwards and form a tower that reaches the tropopause, the barrier between the troposphere and the stratosphere. It will then form an anvil dome or thunderhead (cumulonimbus), a clear sign of an impending thunderstorm. Filmed on a late spring afternoon in Alabama, USA.