Download Chapter 8: AIR MASSES

Chapter 8: AIR MASSES
Answers to the Chapter Review
1.
An air mass is a very large body of air that is characterized by relative uniformity of temperature
and humidity at any given altitude. When an air mass is moving over an area, that region will
probably experience fairly constant weather conditions for perhaps several days, a situation
termed air-mass weather.
2.
A source region must be an extensive and physically uniform region characterized by a general
stagnation of atmospheric circulation.
3.
These regions are not likely to produce air masses because converging winds are constantly
bringing air with unlike temperatures and humidities into the area.
4.
Air masses are classified on the basis of their moisture content (continental versus maritime) and
their temperature (tropical, polar, arctic). Continental polar (cP) is a dry and cold or cool air
mass; maritime polar (mP) is a humid and cool air mass; maritime tropical (mT) is a humid and
warm or hot air mass; continental tropical (cT) is a dry and hot air mass.
5.
Since arctic air masses form over the ice-covered Arctic Ocean, they consistently have moisture
characteristics associated with a continental source region (cA).
6.
The lower case letter k indicates that an air mass is colder than the surface over which it is
passing, whereas the lower case letter w indicates that the air mass is warmer than the surface
below. A k air mass is warmed from below, and thus it is made less stable. Therefore, such an
air mass is often characterized by cumuliform clouds, showers, or thunderstorms, should
precipitation occur, and good visibility except during rain. On the other hand, since a w air mass
is chilled from below, it is more stable. Therefore, if a w air mass has clouds, they will be
stratiform clouds, and precipitation, if any, will be light. In addition, poor visibility and advection
fog are associated with w air masses.
7.
Wintertime cP air masses are normally colder than mP air masses because the continents are
colder than the ocean surface during winter.
8.
Such mechanical or dynamic modifications are usually independent of the changes produced by
surface cooling or heating. Convergence or forceful lifting over a highland barrier will cause the
air mass to become less stable. The same happens when an air mass encounters a low-pressure
system. Conversely, subsidence associated with high-pressure centers acts to stabilize an air
mass.
9.
Maritime tropical and continental polar air masses are most important to the weather east of the
Rockies. Continental polar air masses dominate in winter, often bringing cold waves. Moisture
from maritime tropical air masses is the primary source of much, if not most, of the precipitation
received in the eastern two-thirds of the United States.
10.
Maritime polar air masses from the North Pacific source region are most important to the weather
along the Pacific coast.
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11.
Because there are no major barriers to their movement between their source region and the Gulf
of Mexico. Thus, these cold air masses can sweep with relative ease far southward into the
United States.
12.
Initially, the cP air mass is cold and stable. As it moves across the warmer Great Lakes, it
becomes a cPk air mass. The addition of heat and moisture from the warm water surface renders
the air mass unstable, and heavy lake-effect snow is the result.
13.
Since the general airflow is from west to east, these air masses usually move away from North
America toward Europe.
14.
Maritime tropical air from the Gulf of Mexico.
15.
a.
b.
c.
d.
e.
cT air from northern Mexico and the American Southwest
mT air from the Gulf of Mexico
mP air from the Northern Pacific
mT air from the Gulf of Mexico
mP air from the Northern Atlantic
Answers to the Chapter Problems
1.
See Figure 8-11. A continental polar (cP) air mass is over western Canada and the northwestern
United States. The Boundary goes through Oregon, Idaho, Montana, North Dakota, then into
Canada at the Minnesota border and turns north toward Hudson Bay.
A maritime tropical (mT) air mass is over the eastern and southeastern United States. The
boundary is from the western borders of Texas and Oklahoma, through Kansas, Iowa, and
Wisconsin then north through the upper peninsula of Michigan toward Hudson Bay.
2.
Each station has its maximum snowfall in November. This maximum probably occurs in
November because air temperatures are cold enough to produce snow and are still warm enough
to allow a great deal of moisture to remain in the air.
3.
The area of relatively heavy snowfall east of Pittsburgh and Charleston in Figure 8-5 is probably
due to the higher altitude of the Allegheny Mountains.
4.
The two rainiest months are July and August. The rainiest months occur during the summer
because of the North American monsoon. In the summer, a low-pressure center develops over
Arizona that draws in warm, moist air from the Gulf of California (some moisture may come
from the Gulf of Mexico as well). This supply of moisture, coupled with the convergence and
upward flow of the thermal low, is responsible for the precipitation Albuquerque experiences
during the hottest months.
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