Cumulus thundercloud. Diagram showing the formation of a thunderstorm cloud (white) along a cold front, where cold air (blue arrow) sinks down, while warm air (red arrow) rises and carries moisture upwards. As the warm air ascends it cools and the moisture condenses to form clouds, which may then release the moisture in the form of rain. Cold fronts can lead to the formation of thunderstorms. 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.