Hot air rises when heated by the sun, carrying water vapor into the sky.
As it rises, the hot air mingles with colder air, and the moisture condenses into water droplets. Clouds are created when these water droplets become visible. The droplets increase in size as the cloud grows and eventually become so heavy that they fall as rain. Thunderclouds are large, anvil-shaped masses that can stretch miles across at the base, and reach 40,000 feet or more into the atmosphere. The genesis of lightning is a subject of great theoretical debate, says Dr. Vladimir Rakov of the Lightning Research Center at the University of Florida.