Storms form when warm, moisture-laden air rises rapidly into the atmosphere. As the air rises, it cools and the moisture condenses, falling back to Earth as precipitation.
Storms are fueled by heat and moisture, which is why storm activity, particularly thunderstorm activity, is greater during the summer season. The rising warm air cools and falls, creating a convection current that feeds the storm. This is why storms often form and intensify over water, and then lose intensity or die out completely as they move over land. When conditions are favorable, storms often appear and develop very quickly. Some storm systems last for several hours, producing multiple storms, or cells, over their life cycles.
In many instances, the moisture needed to fuel a storm is actually supplied by a mass of air moving through a particular area. This is why so many storms form at the boundaries where different air masses meet. The famed Tornado Alley is a result of this phenomenon; cool, dry air comes down from the north while warm, moist air comes up from the south. Meeting over the open plains, these air masses mix and are warmed by the sun to produce very intense storms, which often develop into huge, super-cell thunderstorms or even tornadoes.