All thunderstorms are similar in physical makeup, but for purposes of identification, they may be divided into two general groups, frontal thunderstorms and air-mass thunderstorms.

FRONTAL. Frontal thunderstorms are commonly associated with both warm and cold fronts.

The warm-front thunderstorm is caused when warm, moist, unstable air is forced aloft over a colder, denser shelf of retreating air. Warm-front thunderstorms are generally scattered; they are usually difficult to identify because they are obscured by other clouds. 

The cold-front thunderstorm is caused by the forward motion of a wedge of cold air into a body of warm, moist unstable air. Cold-front storms are normally positioned aloft along the frontal surface in what appears to be a continuous line. Under special atmospheric conditions, a line of thunderstorms develops ahead of a cold front. This line of thunderstorms is the prefrontal squall line. Its distance ahead of the front ranges from 50 to 300 miles. Prefrontal thunderstorms are usually intense and appear menacing. Bases of the clouds are very low. Tornadoes sometimes occur when this type of activity is present.

AIR MASS. Air-mass thunderstorms are subdivided into several types. In this text, however, only two basic types are discussed, the convective thunderstorm and the orographic thunderstorm.

Convective. Convective thunderstorms may occur over land or water almost anywhere in the world, Their formation is caused by solar heating of various areas of the land or sea, which, in turn, provides heat to the air in transit. The land type of convective thunderstorm normally forms dur-ing the afternoon hours after Earth has gained maximum heating from the Sun. If the circula-tion is such that cool, moist, connectively unstable air is passing over the land area, heating from below causes convective currents and results in towering cumulus or thunderstorm activity. Dissipation usually begins during the early even-ing hours. Storms that occur over bodies of water form in the same manner, but at different hours. Sea storms usually form during the evening after the Sun has set and dissipate during the late morning.

Both types of convective thunderstorms occur in Florida. The anticyclonic circulation around the Bermuda high advects moist air over the land surface of Florida in its easterly flow.

Thunderstorms off the east coast of Florida at night occur when this easterly flow passes over the warm axis of the Florida current. In those areas where the air is cooler than the water below it, the air is heated and convective currents (lift-ing) begin. Any nocturnal cooling of the easterly flow aloft aids in establishing the unstable lapse rate necessary for thunderstorm development. After sunrise, the air is heated and becomes warmer than the water, thereby destroying the balance necessary to sustain or build similar storms. As the day progresses, the land surface becomes considerably warmer than the air. Con-vective currents again result, and Floridas com-mon afternoon thunderstorms are observed. After sunset the land cools, convective currents cease, and the thunderstorms dissipate. The apparent movement of the storms to sea at night, and to shore during the day, is in reality the reforma-tion of storms in their respective areas. As a general rule, convective thunderstorms are scat-tered and easily recognized. They build to great heights, and visibility is generally excellent in the surrounding area.

Orographic. Orographic thunderstorms form in mountainous regions, particularly adja-cent to individual peaks. A good example of this type of storm occurs in the northern Rocky Mountain region. When the circulation of the air is from the west, moist air from the Pacific Ocean is transported to the mountains where it is forced aloft by the upslope of the terrain. If the air is conditionally unstable, this upslope motion causes thunderstorm activity on the windward side of the mountains. This activity may form a long, unbroken line of storms similar to a cold front. The storms persist as long as the circulation causes an upslope motion. They tend to be more frequent during afternoon and early evening when convective lifting coincides with the mechanical lifting of the terrain.

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