The following statements summarize and describe many of the key terms and concepts 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 usually more intense and of shorter
duration than is precipitation associated with a warm front.
- 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. These bearers of stormy weather, which last
from a few days to a week, have a counterclockwise circulation pattern in the
Northern Hemisphere, with 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.