LITMIR.BIZ

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case you haven’t noticed, the energy that radiates back from the surface travels as long infrared waves, which you and I can’t see. Invisible it might be, but this form of radiation is more important than you might think. The atmosphere, which lets most of the incoming short-wave sunlight pass right through without absorbing it, catches a lot of the rebounding long-wave heat energy and keeps it around. This produces the so-called greenhouse effect that is discussed in detail in Chapter 14.

      BRING IN THE CLOUDS

      Clouds love this long-wave infrared radiation. The tiny water particles in them soak it right up.

      Have you ever noticed that a cloudy night often is warmer than a clear one? Those clouds are absorbing that radiation coming off the Earth and radiating a lot of it back toward the ground, acting like a big infrared blanket over you.

      A cloudy day can be a different story, depending on the season. In spring and summer, the same clouds that kept you warm at night now are preventing the sunlight from reaching you. More often than not, a calm cloudy summer day is cooler than a calm sunny day. In winter, clouds during the day often make temperatures near the ground warmer than on a clear day.

      HOW TO CAUSE A STORM

      Did you know there are two forms of heat?

      One is the kind of heat that you feel on your arm, say, when you exercise or hang out in the sunshine. This they call sensible heat, because you can sense it. (This sounds pretty sensible to me, although I wouldn’t spend too much time out in that sunshine.)

      The other kind is latent heat. This is heat that is released or absorbed when things like water change phase, or form, between vapor and liquid and ice. They call it latent because it is stored away, or hidden. (They could have called it insensible, you know, but nobody asked me.)

      This idea sounds a little tricky at first, but really it’s no sweat. Look at it this way:

      When you perspire, your body is working on getting rid of excess sensible heat. The sweat on your arm evaporates, converts from liquid water to gaseous vapor. This process of conversion from liquid to gas state absorbs heat, and the coolness you feel is the sensible heat being converted to latent heat. The heat that left your body is stored away in the molecules of that little bubble of air that just lifted off from your arm.

      Now follow that water vapor off of your arm as it rises up higher and higher into the sky and forms a cloud. When it does this, it converts itself back into liquid, tiny water droplets, and gives the heat it took off your body back to the atmosphere. Oops, things look a little unstable up there. A storm is brewing! Now see what you’ve done!

This invisible long-wave radiation that is given off by the Sun-warmed Earth is more important to weather than direct sunlight. Somebody who has spent the day in the sunshine may find it hard to believe, but a weather scientist will tell you: The energy radiating back up into the atmosphere from the surface of the Earth as long-wave radiation has more direct effect on weather processes than the short-wave energy that comes directly through the atmosphere as sunshine.

      A contagious convection

      Heat that is moving from the surface of water or land warms the air just above it in a process of direct transfer known as conduction. This is the way the icy cold of a glass or the boiling heat from a cup travels up the handle of a metal spoon, for example. And just around the metal spoon handle, a thin layer of air is absorbing some of the heat.

From this thin layer of air at the surface, the heat energy finds its way into higher levels of the atmosphere through a process known as convection, the vertical mixing of liquid or gas of different temperatures. Convection is what happens when a pot of water boils.

      

Some of this air mixing happens through the mechanical forcing of wind. This is referred to as forced convection. Blowing near the surface, swirling eddies in the flowing air carry the heat up into the sky. Two general rules apply: the faster the wind, the greater this kind of convection. Also, the more uneven the surface — the bigger and more numerous the eddies — the greater this kind of mixing.

      Another kind of vertical mixing known as free convection depends on buoyancy — the ability of warmer air to rise in cooler air. In the atmosphere, a kind of bubble of warm air is formed near the surface and floats up to higher altitude, above the cooler, denser air around it, much like a hot-air balloon would do. As it rises higher and higher, the bubble of air expands, and as it expands, it cools. This kind of rising and falling of air of different temperatures and densities is going on all the time.

      The process of free convection can be especially noticeable on a warm summer afternoon. The Sun is heating the ground and the heat from the ground is quickly warming the air just above it. Before long, a rising column of warm expanding air is formed. These are the thermal updrafts that soaring birds ride on a warm day.

      If conditions are right, if the air bubble contains enough moisture and the surrounding air is colder than the bubble of air, a cloud can eventually form when the rising air gets cold enough for its water vapor to condense into tiny liquid droplets or even ice crystals. (For more about cloud formation, see Chapter 6.) This condensation process gives off still more heat, called latent heat. This latent heat plays a major role in the in the formation of clouds and storms. (See the sidebar, “How to cause a storm.”)

The Big Picture

      

You want to know what’s really behind all that turbulent mess you think of as weather? (No, my people at the Go Figure Academy of Sciences have looked into it, and they tell me it’s not the government.) Do you want the Big Picture? Well, now it can be told. Believe it or not: It all has to do with the way the solar system is put together. (Hey, you wanted the Big Picture!)

When you really get down to it, three large facts of life in this particular reach of the solar system are responsible for the behavior of Earth’s atmosphere. You can see them all in Figure 3-5. It has to do with the orbit of the Earth around the Sun. It has to do with the way the planet rotates on its own axis, the way it spins like a top. And it has to do with the fact that it is not spinning straight up and down, but at a tilt. Also, notice how the Earth is closest to the Sun on January 3, in the middle of the Northern Hemisphere’s winter. Go figure!

      FIGURE 3-5: The Big Three behind the weather on Earth: its yearlong orbit around the Sun, its tilt that gives the year its seasons, and its daily rotation.

      Long live the revolution!

      If you told me that it takes a year for the Earth to travel completely around the Sun, and that a year is 365 days, you would be accurate enough for most purposes. But I might not want to set my clock by yours. Did you remember Leap Year — the fact that you add a 29th day to February every four years? This makes up for the fact that the complete revolution of Earth’s orbit around the Sun actually takes 365¼ days.

      There’s something else about Earth’s orbit of the Sun that is a little, well, irregular. If you look at it closely, you will see that it is not really a circle — that is, the Sun is not

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