of course. But if you want something to blame it on, blame the weather on what happens deep within the interior of a star 93 million miles away!
Moving Sun’s energy
The atmosphere is always in the process of converting the Sun’s energy from one form to another and moving it from place to place. Behind all the motion, and commotion, is a complicated exchange of energy between the atmosphere and the Earth. Things are out of balance, and the system is trying to even them out. Weather is the name we give to the atmosphere’s turbulent efforts to balance the cold with the hot, the dry with the wet. The atmosphere deflects more of the Sun’s radiation than it absorbs, and Earth’s surface has a solar energy surplus. This exchange process is what keeps the atmosphere from becoming unbearably cold and the ground you and I stand on from getting unbearably hot.
While weather watchers like you and I focus on the sky, on the rays of the Sun beating down on us and on the rain or snow falling from the clouds, a weather scientist sees things as part of an energy transfer that is moving from the ground up. A winter storm at the seam between two air masses, which this chapter’s previous section, “News from the Fronts,” is about and Chapter 8 describes more fully, is transferring energy from the surface to the atmosphere. So is a summer thunderstorm, which Chapter 10 describes. And so is an autumn hurricane, which Chapter 7 details. Through conduction and convection, they are all moving one form of heat or another from a warmer region to a cooler one.
Later in this chapter, the section, “The Big Picture” explains why the atmosphere never succeeds at this process of balancing the Sun’s energy around the world. Figure 3-4 shows what happens to the radiation from the Sun striking Earth.
FIGURE 3-4: Here’s what happens to the radiation from the Sun once it reaches the atmosphere.
REFLECTING ON ALBEDO
Some sunshine is missing. Nobody leave the room.
About half of the solar energy that reaches Earth’s atmosphere ends up getting absorbed at the surface. There it gets converted into invisible long-wave radiation. When it re-enters the atmosphere sooner or later, it helps make weather. Another 20 percent gets absorbed by the atmosphere and clouds on the way down.
So what happens to the other 30 percent of the sunshine? People who have been put on the case say it gets lost to scattering, when sunlight rays collide with air molecules or tiny dust particles are reflected back off of bright surfaces. The brighter the surface, the more light it reflects (and the less it absorbs). This is why a white shirt is cooler than a dark one on a summer day. The percentage of light that a surface reflects back, rather than absorbs, is a property that scientists call albedo.
Albedo is a big deal. For example, 20 percent of the incoming sunshine bounces right off the bright white cloud tops and heads back toward space. About 4 percent is reflected back from Earth, but there are big differences in the albedo of different surfaces. It ranges anywhere from 95 percent for fresh snow to 2 percent for calm water.
Looking absolutely radiant!
Don’t look now, but waves of energy are radiating all around you. The fact is, everything that has a temperature above absolute zero (–459.67 degrees Fahrenheit/–273 degrees Celsius) is giving off at least some waves of radiation. Your body, for example. Even your Weather For Dummies book. This radiation is an important part of the process of turning the Sun’s energy into weather.
When it comes down from the Sun, however, most of the energy arrives as powerful short-wave radiation, including the spectrum of light that you can see, and this passes right through the atmosphere and strikes the surface of the Earth. Depending on the kind of surface it hits, it bounces back or is absorbed. It all depends on color and surface texture and other properties of the surface. Notice how hot a black asphalt parking lot gets on a summer afternoon, absorbing the heat, and yet, how quickly it cools, or radiates it away, overnight.
People who plan cities and buildings are studying these different heat-absorbing and radiating properties of materials to make downtowns and neighborhoods more energy-efficient and comfortable places to be.
In 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
