Thunderstorms develop when cold upper air sinks and warm moist air rises. As the warm air rises, storm clouds develop. These clouds make the thunderstorm, which brings strong winds, lightning, hail and rain. Thunderheads may be miles across at the base and reach heights of 40,000 feet or more. The typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. They most often occur during the afternoon and evening hours. No matter their size, all thunderstorms can be dangerous. In addition, tornadoes and flash floods can be caused by these storms.