LITMIR.BIZ

Скачать книгу

GOES THE MOON

      Sailors and poets and ancient thinkers all favored the idea that the moon has a lot to do with how things go here on Earth. A “ring around the moon” was seen as a sign of impending storminess, for example, but none of it is the moon’s doing.

      That ring or halo is moonlight — sunshine reflecting off of the moon’s surface — scattering as it encounters thin clouds in the atmosphere. They might be high in the atmosphere, those clouds, but they are nowhere near the moon.

      But the moon has this important indirect impact on weather: The tug of its gravity makes the tides of oceans and very large lakes rise and fall, and during powerful coastal storms, high tides can make the difference between inconvenience and natural disaster.

      Stationary fronts

      A stationary front, as you might expect, isn’t going anywhere, at least for the time being. But don’t let its lack of motion fool you. A stationary or slow-moving front can mean real trouble. A lot of weather can be happening around a front that isn’t moving. Sometimes, when warm humid air travels over the frontal boundary, rainfall can occur over large areas, and because the front isn’t moving, the rain may stay long enough to cause flooding.

      Occluded fronts

      As a winter storm progresses, the large wedge of warmer air between the warm front and the cold front gets narrower. The cold front travels faster than the warm front, and beginning near the low-pressure center of a mature storm, it eventually overtakes it. This new boundary is called an occluded front. On one side is cold air from behind the cold front. On the other is cool air that the warm front was flowing over. Above the front is warm air that has been cut off from the ground.

Here Comes the Sun

Weather is the Sun’s doing. It might be hard to believe when you are being hammered by a cold winter storm, but the Sun, the star of the solar system, is the driving force behind all this weather, the commander in chief of all these air wars. The heat energy — the solar energy — radiating from this star is the fuel that drives it all, everything I describe in Weather For Dummies. If it were not for the Sun’s warming of Earth and its atmosphere, the planet would be frozen solid, a ball of ice. (And nobody would be around to read — or write — about the weather.) The Sun’s energy is the fuel, and weather is the result of the huge temperature differences between the Equator and the poles. These temperature differences are partly Earth’s doing, 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.

      

Through heat-transfer processes that weather scientists call conduction and convection, the radiation surplus is continually moving from the surface up into the atmosphere. The heat is conducted directly from the surface to the thin layer of atmosphere above it. You might say that the convection process takes it from there, mixing it through winds and other weather processes among the higher layers of atmosphere.

      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.

Скачать книгу