As the day progresses, the land surface becomes considerably warmer than the air. 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 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.