A thunderstorm is a localized storm accompanied by lightning and thunder. It may also have gusty winds and often brings heavy rain. Some thunderstorms can also bring tornadoes and/or hail. During winter, localized heavy snow showers may also have thunder and lightning. And, in the western United States in summer, thunderstorms may be "dry," bringing lightning and thunder but no rain. Thunderstorms are produced by cumulonimbus clouds and form when air rises rapidly. Sunlight can drive the rising air in these clouds, or air can rise rapidly for other reasons, i.e., cold fronts can push warm, moist air upward as they move across a region, or air blown into a mountain range can be forced to rise. This article describes the basic causes and components of thunderstorms.