AIR MASSES
Learning Objectives
After reading, studying, and discussing this chapter, students should be able to do the following:
1. Describe what an air mass is and describe the concept of air-mass weather.
2. List two criteria that an air-mass source region must meet.
3. Discuss the basis of air-mass classification.
4. Describe the ways that air masses are modified.
5. List the three factors that the daily weather we experience depends on.
6. Discuss the weather conditions associated with cP, mP, mT, and cT air masses that influence North America.
7. Define the terms listed in the vocabulary review.
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.
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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.
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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.
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. cT air from northern Mexico and the American Southwest b. mT air from the Gulf of Mexico c. mP air from the Northern Pacific d. mT air from the Gulf of Mexico e. mP air from the Northern Atlantic
Answers to the Chapter Problems
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1. See Figure 8-11.
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 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.
Suggested Activity
1. On a map of North America and the surrounding oceans, have students label the locations of the various air-mass source regions, including their temperature and humidity characteristics and the directions in which they are likely to move.
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