A "maritime tropical" is an air mass that originates over tropical waters or moves across an ocean. In North America, they most often affect the Southeastern United States.
A maritime tropical air mass is full of warm, moist air. During the winter, these air masses stay close to the equator and Southern U.S. During the summer, however, they often spread up the Eastern Coast. They usually bring unstable air into the atmosphere, which leads to thunderstorms; this is especially true when the air mass encounters a cold front. The Western Coast of the United States sometimes experiences maritime tropical air that forms over the Pacific Ocean and moves into California. They bring thunderstorms, fog and low-lying clouds to the area.
Maritime air masses are different from continental air masses because they form over water instead of land. Maritime polar air masses also form over water, but they bring cooler and drier weather rather than warm and moist weather. Britain experiences maritime tropical air masses that originate in the Azores. They cause mild and damp weather in the British Isles during the winter, and warm, muggy weather during the summer. In North America, continental tropical air masses form over Mexico and move north, bringing clear, dry weather.