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.
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
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 in the center of Earth’s orbital path. Instead, it is off to one side. The shape of the orbit is elliptical, which means that at some times during the year the Earth is actually closer to the Sun than at other times.
This state of affairs might lead you to think — as some people do — that Earth’s elliptical orbit is responsible for the fact that some times of year are warmer than others — that summer might be caused by the fact that the Earth and Sun are closest together at that time of year. This is a completely mistaken idea, and you should wash it out of your mind immediately. In fact, I’m sorry I brought it up!
If you have any doubts, consider this fact: The Earth is closest to the Sun every year on January 3. I don’t know about you, but I live in California, and while we have had some pretty nice January days, January 3 has never felt much of anything like summer. If I lived in the Southern Hemisphere, south of the Equator, where January 3 is in the summertime, I might think differently about this, of course. But still I would be wrong. Earth being closest to the Sun at that time of year is not responsible for the fact that it is summertime there either.
Here’s what it means: On or about January 3, the Earth and the Sun are a mere 91 million miles apart. Six months later, on July 3, at the opposite side of the elliptical orbit, when they are farthest apart, the distance has stretched to 94 million miles. This is about a 3 percent difference in the Earth-Sun distance from one time of year to another.
It has an impact on the intensity of sunshine reaching Earth, no doubt about it. Scientists have figured out that Earth gets 7 percent more heat energy from the Sun on January 3 than it does on July 3. This is because on July 3, even though it is mid-summer in the Northern Hemisphere, the sun’s rays are traveling a little farther and so are slightly more spread out than they are on January 3. But this small difference does not account for the seasons. The angle that they strike a particular place on Earth makes a lot more difference to the intensity of the Sun's rays. As Figure 3-6 illustrates, the angle is what the seasons are all about.
