Читать книгу Weather For Dummies - John D. Cox - Страница 42

An air mass is a widespread body of air that has a uniform look to it. Across hundreds if not thousands of miles, all this air looks and acts pretty much alike. This is because it has been parked over a particular region of the Earth long enough to absorb some of its important qualities. It has picked up from the surface a certain temperature and humidity, or moisture content, and like in any good army, these characteristics are fairly evenly distributed. There are no real storms or battles going on or even strong winds — this is all the same army, after all — and pressure, like morale, is fairly high.

Just as different regions of the world have certain characteristics, so do the air masses that form over them. The big differences that make dramatic weather are temperature and humidity — they are warm or cold and moist or dry.

 Continental air masses form over land and are dry.

 Maritime air masses form over an ocean and are moist.

 Polar or Arctic air masses are cold.

 Tropical air masses are warm.

The action begins when an air mass moves from its place of origin, usually in response to winds in the upper atmosphere. (To read about what causes wind, check out Chapter 5.) While weather scientists now think of the winds high in the atmosphere as the driving forces behind the storms, the weather in your face still has the look and feel of a battle of air masses. In the Northern Hemisphere, the half of the world north of the Equator, you can be sure that a cold air mass is moving down out of the cooler regions of the north and a warm air mass is moving up from the warmer regions in the south.