Weather For Dummies. John D. Cox

      FIGURE 3-1: Imaginary lines called latitudes divide the world into the Tropics, the polar regions, and between them, the mid-latitudes.

      You might think things would be pretty comfortable in the middle of the house, the middle latitudes, where most people live in the world. Not too hot, like Goldilocks said, and not too cold. And certainly it’s true that, on average, the mid-latitudes are less extreme environments to live in. But there’s a catch, of course. In case you haven’t noticed, it is in the middle latitudes where you and I live that a lot of the masses of northbound tropical warm air and southbound cold polar air come together. And when they do, it’s not a pretty picture. In fact, it can get messy.

Where the Armies Mass

      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.

      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.

      A WEATHER WAR ZONE

      Every place has its own dangerous weather at one time of the year or another. But did you know that the continental United States gets more violent weather than anyplace else on Earth? This came as a big surprise to early American settlers.

      In a typical year, the National Weather Service reports the continental United States can expect these violent weather events:

      Why all these extreme weather events? It’s like they say about the real estate business: The three most important things are location, location, and location.

As Figure 3-2 illustrates later in this chapter, one region or another of the continental United States is in pathway of a variety of moist and dry and warm and cold air masses coming at it from all directions.

      Winter air masses

Maritime polar air masses regularly sweep over the West Coast from out of the northern Pacific Ocean, bringing cool, moist air that dumps heavy snow in the western mountains. By the time this air crosses east of the Rockies, it is relatively dry. It warms as it blows down the eastern slopes of the mountains and sometimes brings fair weather and warming temperatures to the plains. Maritime polar air also originates in the North Atlantic and occasionally sweeps southwestward into New England and the mid-Atlantic states, bringing rain and snow.