LITMIR.BIZ

      FIGURE 3-7: Here is a close-up view of Earth’s 23.5-degree slant and how it affects the distribution of sunlight in the course of its yearlong revolution around the Sun.

      These dates are often said to be the “first official days” of the various seasons, but take another look. Is December 21 really the first day of winter where you live? As my people say at the Go Figure Academy of Sciences, this is the kind of thing that can happen when you send an astronomer to do a meteorologist’s job! In most parts of the United States, if you haven’t been wearing your winter coat before December 21, you haven’t been keeping your promises to your mother. Likewise, by June 21, the day of the year when the Sun’s rays beat down directly over more of the Northern Hemisphere than any other, summertime has already become a pretty familiar feeling.

      In the middle latitudes of the Northern Hemisphere, weather scientists generally think of the winter season as the months of December, January, and February, spring as March, April, and May, and so on. (From winter through autumn, Part 3 goes into the details of the weather effects of these seasonal changes.)

      Spin of the day

      You and I are part and parcel of Earth’s motions in space, and the planet’s atmosphere also is going along for the ride. Although Earthlings are traveling in a yearlong revolution around the Sun, speeding through space, they have no sensation of this movement. Thank goodness for that! Talk about being blown away!

      Also, people have no sensation of dizziness even though the planet is spinning like a top in a rate of rotation that completes itself every 24 hours. A person standing at the Equator not only is traveling through space along with the planet, but at the same time is spinning with the planet at more than 1,000 miles per hour. It’s like a whirligig carnival ride, revolving on one level and spinning on another, and I’m getting a little queasy just thinking about it.

      Such is the fickle pace of many daily weather events. The hot summer afternoon can conspire with the moist air to produce a violent local storm of lightning and thunder and hail. The ground and the town below it can be left in a mess in an hour or two, and before long, the sky can show not the slightest sign of what happened.

      Earth’s rotation is responsible for some very large and powerful weather-related motions in the atmosphere. For example, weather patterns in the middle latitudes move from west to east because of Earth’s spin. It is responsible for the west-to-east direction of the powerful polar jet streams and for the prevailing global winds, such as the tradewinds and the mid-latitude westerlies, which Chapter 5 describes in detail.

Putting on Airs

The atmosphere, the weather’s home, begins at the tip of your toes and extends some 80 miles up, more or less. That may sound like a pretty deep sky, but relative to the size of the Earth, its thickness is less than a rind on an orange. And the layer of atmosphere where all the weather takes place is much thinner still — only about 10 miles thick — more like the skin on a peach. Only this skimpy layer, this peach fuzz, contains enough of the ingredients in the right proportion that you and I need to breathe. Does it strike you as odd, by the way, that people so seldom give this vital substance much thought? Go figure. So what exactly is this atmosphere — this precious stuff called air?

      Do I smell gas?

The atmosphere is a thin envelope of gases surrounding Earth. Figure 3-8 lays out its contents, within about 50 miles of the surface, and quickly you can see that mostly it is nitrogen and oxygen. No, you can’t really smell the gases that make up the air. This mixture is odorless and tasteless. And you can only see it when it contains water drops or something else — when it’s dirty. Then you smell it and you can see it, even if you don’t want to. (Chapter 14 has the low-down on air pollution.)

      FIGURE 3-8: The gases that make up the atmosphere.

      While it makes up some 78 percent of the atmosphere, nitrogen is not in the weather-making business. It makes the natural nitrogen that is absorbed by the soil and is essential for the growth of plants. It does combine to form nitrous oxide, an important ingredient in smog, which is described in Chapter 14.

      The following sections describe the gases that are most important to weather.

      Oxygen

      Oxygen, the gas that sustains all life on Earth, is constantly being recycled between the atmosphere and the biological process of plants and animals. It combines with hydrogen to form water, which in its gaseous state, water vapor, is the most important component of the atmosphere as far weather is concerned.

The form of oxygen known as the gas ozone also is vital to life on Earth. Ozone forms a thin layer in the upper stratosphere that filters out harmful ultraviolet radiation. Chapter 14 describes what happens when this crucial layer is depleted — when an ozone hole forms — and what the world’s governments are doing about it.

      Water vapor

      If there is one substance in the atmosphere more involved with weather than any other, it is the gaseous form of water. At its most concentrated, water vapor makes up only 4 percent of the atmosphere, and yet, almost no important weather takes place without it.

      Without water vapor to condense into droplets of water or ice crystals as air rises and cools, no clouds would form in the sky.

      Without water vapor, there would be no precipitation — no rain and no snow. The cycling of water through the environment, as described in Chapter 4, would come to a screeching halt without water vapor in the atmosphere.

      The condensation of water vapor leads to the release of latent heat, which is described earlier in this chapter in the sidebar “How to cause a storm.” Latent heat supplies the atmosphere with the energy that is important in the formation of storms, especially thunderstorms and hurricanes.

      Water vapor also is a potent gas in the greenhouse effect, which is outlined in more detail in Chapter 14. Like the glass top of a greenhouse, it absorbs infrared heat emissions from Earth’s surface, preventing it from radiating back into space.

      Carbon dioxide

      Like water vapor, the gas carbon dioxide has powerful greenhouse effects, trapping outgoing heat radiation, a process that is natural and beneficial — up to a point.

      Carbon dioxide is constantly recycled through biological process of animals and

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