| Summary: | An air mass is a regional-scale volume of air with horizontal layers of uniform temperature and humidity. Air masses form during episodes of high pressure when weak winds allow air to remain for several days over a flat area with uniform surface characteristics. The characteristics of the underlying surface determine the characteristics of the air mass, which is given a two-letter identifier. Air masses are identified by their locations of origin (maritime “m” or continental “c”) and by their characteristics (tropical “T” or polar “P”). Tropical air masses form in high pressure areas in warm, tropical regions. When a tropical air mass is formed over oceans (mT), it is warm, moist, and usually unstable. When formed over land (cT), it is hot and dry, with unstable air near the surface and stable air aloft. Polar air masses form in high pressure areas in the polar and subpolar regions. A polar air mass that forms over water (mP) is cool, moist, and unstable. A polar air mass that forms over land (cP) is cold, dry, and stable. An extremely cold polar air mass that forms in winter over arctic ice and snow surfaces is called an arctic air mass (cA). The distinction between arctic and polar air masses is not always clear because an arctic air mass that travels over a warm surface may be warmer near the surface than a polar air mass, although it is still colder aloft. Source regions for air masses and typical trajectories affecting North America are shown in figure 6.1. Polar air masses that originate over the flat, ice- and snow-covered regions east of the Rocky Mountains in northern and central Canada and Alaska, and arctic air masses that originate over the ice-covered Arctic Ocean influence winter weather. The midlatitudes are not a good air mass source region. The exposure to traveling weather systems is too great, the range of temperature and humidity too wide, and, in the United States, the topography is too varied. Instead, the midlatitudes are a region where clashing air masses meet. Cold air masses are usually ...
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