Weather is greatly influenced by atmospheric pressure systems. High pressure and low pressure centers are indicators of the type of weather to be expected. Winds blow in a counterclockwise direction around a low pressure center and in a clockwise direction around a high pressure center. Closely spaced systems indicate a steep pressure gradient and high wind speeds; when the systems are farther apart, the wind speeds are lower. A trough line may develop between two low pressure areas, and a ridgeline may develop between two high pressure areas. In general, cloudy or rainy weather is associated with a low pressure center, and clear, sunny weather accompanies areas of high pressure.
Differences in pressure are caused by air masses moving across the country. An air mass is a large body of air that has a more or less uniform temperature and moisture content throughout the mass. Air masses that have been over water for a period of time may contain large amounts of moisture, whereas those originating over land areas are usually dry.
The weather in the United States is influenced by five air masses, Figure 16.1. The following section lists and briefly describes these air masses.
1. Tropical maritime (mT): This air mass forms off the Gulf of Mexico where it is subject to tremendous heating by the sun. This heating causes evaporation from the ocean, resulting in a warm, moist air mass. This mass generally moves north and west into the central region of the country. This mass contributes the largest amount of moisture to the central and eastern regions of the United States. Tropical maritime air masses also form seasonally of the southern coast of California.
2. Tropical continental (cT): The tropical continental forms over the Mexican countryside. The cT is subject to tremendous heating by the sun; but because it forms over land, it is dry. It tends to move north into the central plains. When a cT moves into a region dominated by a mT, a dry line can develop at the boundary line between the two air masses. This condition is famous for the development of severe thunderstorms.
3. Polar maritime (mP): This air mass forms over the polar regions of the Pacific and Atlantic Oceans. They are very cool and usually saturated. Consequently any additional cooling can produce precipitation. They are notorious for producing fog, drizzle, cloudy weather and long lasting, light to moderate rain. The Pacific mP occurs more frequently and tends to move south and east into the central plains. As it is forced up over the Rocky Mountains it loses most of its moisture and takes on the characteristics of a continual air mass. The Atlantic mP occurs less frequently and tends to follow the east coast.
4. Polar continental (cP): This mass forms over the central plains of Canada. This area of Canada has long winter nights and a large amount of radiation cooling resulting in a very cold, dry air mass. This air mass also tends to move very slowly, and consequently usually is very cold. Consequently this air mass does not produce very much precipitation. The exception is along the boundary line between the cP and a mT. In this situation the rapid cooling of the mT caused by the cP can product localized intense thunderstorms. Its normal movement is south into the Great Plains.
5. Superior (S): The superior air mass is unique because it forms at high altitudes over the southwestern desert and occasionally descends to the surface. It is usually very hot and dry.
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