Weather For Dummies. John D. Cox

of them and warm fronts with semicircles along their leading edge. Areas of precipitation are often designated with hatch-marks or shaded sections. Some temperature readings often are included, and occasional arrows will point out the directions of surface winds. Chapter 3 goes into more detail about these weather features.

      FIGURE 2-2: A typical weather map showing features and symbols that are common to most simplified maps used in televised forecasts today.

Chapter 3

      Behind the Air Wars

      IN THIS CHAPTER

      

Watching air masses amass and their fronts do battle

      

Seeing how the Sun’s energy powers the weather

      

Sorting out the effects of orbit and tilt and rotation

      

Composing an atmosphere

      If there is a personality that describes the atmosphere, the blanket of air where all the weather takes place, you might think of it as flighty or fickle. Do you know people who change their minds a lot? Who often seem to be repeating the ideas of the last person they were with? That’s the atmosphere all over. (You want to scream sometimes: “Make up your mind!” Does it help?)

      At the Go Figure Academy of Sciences, weather experts describe this maddening characteristic in polite, five-dollar terms like instability and turbulence and chaos. Frankly, the word unbalanced comes to mind, if you get my drift.

      As any weather forecaster can tell you, the atmosphere is unreliable — here today, gone tomorrow, as the saying goes, blowing hot and cold. You think you know it when you go to bed at night, and then, poof! — as soon as the Sun comes up, there’s a completely different character. This chapter is all about the things that make the atmosphere and its weather the way it is — so changeable.

I Don’t Like Your Latitude!

Don’t be too hard on the atmosphere. (After all, it’s the air you breathe. Without it, you’re sunk.) Imagine trying to live in a house where it’s always hot at one end and always cold at the other. That’s the situation the Earth’s atmosphere finds itself in. You don’t have to be an Eskimo or a Pacific Islander to figure out where the warm spots and the cold spots are, but it helps.

The low latitudes, around the Equator, get a lot of warm sunshine, and the high latitudes, around the poles, get very little. (Figure 3-1 shows the layout of the imaginary lines around the Earth called latitudes.) This temperature difference is no small matter. On the same day, it can reach 120 degrees below zero overnight in Antarctica and 120 degrees above zero in a subtropical desert. The atmosphere has to deal with these huge temperature differences all the time. They have a lot to do with what weather is all about.

      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.

Those big storms with the long lines that twist together into a whirling low-pressure system on the weather map are called mid-latitude cyclones, or sometimes, extratropical cyclones. You might think of these storms as battles between opposing armies of hot air and cold air. The lines between them are very appropriately referred to as fronts — as in battlefronts. Weather scientists used to think of these storms exactly this way, although now the picture in their minds is more complicated. (The section later in this chapter, “News from the Fronts,” goes into more detail about these clashes.) Nobody who has lived through the devastation of a flood or a blizzard or an especially severe winter needs to be reminded that weather unleashes powerful forces that can do terrible things. You and I are bystanders to all of this, and the only thing to do when a battle is raging is to try to stay safe, warm, and dry.

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.

       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.

      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:

       Roughly 10,000 severe