Weather: Dragons of Chaos The Nature of Things : Season 40 Episode 14

Violent thunderstorms develop in the heat of spring and summer. The heat quickly draws large amounts of water vapour into the air. As the cloud grows, it rises high into the atmosphere. Eventually it hits a layer of cold air -- the boundary between the troposphere and the stratosphere. Blocked from rising, the cloud spreads into an anvil shape. Within the cloud, the droplets combine and grow until gravity draws them back to Earth. It takes a million droplets to make a single rain drop.