Thunderstorm clouds are shaped wherever there is enough upward motion, unsteadiness in the vertical, and wetness to produce a deep cloud that reaches up to levels somewhat colder than freezing. These circumstances are most often met in summer. Lightning occurs less commonly in the winter because there is not as much shakiness and moisture in the atmosphere as there is in the summer. These two ingredients work mutually to make convective storms that can produce lightning. Without volatility and moisture, strong thunderstorms are unlikely. Lightning originates around 15,000 to 25,000 feet above sea level when raindrops are carried upward until some of them convert to ice.