Weather Patterns and Severe Storms
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The following statements summarize the primary objectives presented in the chapter.
- An air mass is a large body of air, usually 1600 kilometers (1000 miles) or more across, which is characterized by a sameness of temperature and moisture at any given altitude. When this air moves out of its region of origin, called the source region, it will carry these temperatures and moisture conditions elsewhere, perhaps eventually affecting a large portion of a continent.
- Air masses are classified according to 1) the nature of the surface in the source region and 2) the latitude of the source region. Continental (c) designates an air mass of land origin, with the air likely to be dry; whereas a maritime (m) air mass originates over water, and therefore will be humid. Polar (P) air masses originate in high latitudes and are cold. Tropical (T) air masses form in low latitudes and are warm. According to this classification scheme, the four basic types of air masses are continental polar (cP), continental tropical (cT), maritime polar (mP), and maritime tropical (mT). Continental polar (cP) and maritime tropical (mT) air masses influence the weather of North America most, especially east of the Rocky Mountains. Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States.
- Fronts are boundaries that separate air masses of different densities, one warmer and often higher in moisture content than the other. A warm front occurs when the surface position of the front moves so that warm air occupies territory formerly covered by cooler air. Along a warm front, a warm air mass overrides a retreating mass of cooler air. As the warm air ascends, it cools adiabatically to produce clouds and frequently light-to-moderate precipitation over a large area. A cold front forms where cold air is actively advancing into a region occupied by warmer air. Cold fronts are about twice as steep and move more rapidly than do warm fronts. Because of these two differences, precipitation along a cold front is more intense and of shorter duration than is precipitation associated with a warm front. A stationary front forms when the air flow on both sides of a front is almost parallel to the position of the front and the surface front does not move. An occluded front occurs when an active cold front overtakes a warm front and the warm air is forced aloft.
- The primary weather producers in the middle latitudes are large centers of low pressure that generally travel from west to east, called middle-latitude cyclones. Middle-latitude cyclones originate along a front where air masses (typically continental polar to the north and maritime tropical to the south) are moving in opposite directions. The frontal surface often takes on a wave shape, which becomes more pronounced as the cyclone matures, and a cold and a warm front evolve. Eventually the cold front catches up to the warm front, a process called occlusion, and the storm comes to an end. Middle-latitude cyclones, which are often the bearers of stormy weather, last from a few days to a week, have a counterclockwise circulation pattern in the Northern Hemisphere, and an inward flow of air toward their centers. Most middle-latitude cyclones have a cold front and frequently a warm front extending from the central area of low pressure. Convergence and forceful lifting along the fronts initiate cloud development and frequently cause precipitation. As a middle-latitude cyclone with its associated fronts passes over a region, it often brings with it abrupt changes in the weather. The particular weather experienced by an area depends on the path of the cyclone.
- Thunderstorms are caused by the upward movement of warm, moist, unstable air, triggered by a number of different processes. They are associated with cumulonimbus clouds that generate heavy rainfall, thunder, lightning, and occasionally hail.
- Tornadoes, destructive, local storms of short duration, are violent windstorms associated with severe thunderstorms that take the form of a rotating column of air that extends downward from a cumulonimbus cloud. Tornadoes are most often spawned along the cold front of a middle-latitude cyclone, most frequently during the spring months.
- Hurricanes, the greatest storms on Earth, are tropical cyclones with wind speeds in excess of 119 kilometers (74 miles) per hour. These complex tropical disturbances develop over tropical ocean waters and are fueled by the latent heat liberated when huge quantities of water vapor condense. Hurricanes form most often in late summer when ocean-surface temperatures reach 27°C (80°F) or higher and thus are able to provide the necessary heat and moisture to the air. Hurricanes diminish in intensity whenever they 1) move over cool ocean water that cannot supply adequate heat and moisture, 2) move onto land, or 3) reach a location where large-scale flow aloft is unfavorable.