The Forgiving Air: Understanding Environmental Changeby Richard C. J. Somerville, Richard C. Somerville
The Forgiving Air is an authoritative, up-to-date handbook on global change. Written by a scientist for nonscientists, this primer humanizes the great environmental issues of our timethe hole in the ozone layer, the greenhouse effect, acid rain, and air pollutionand explains everything in accessible prose. A new preface takes into account/i>
The Forgiving Air is an authoritative, up-to-date handbook on global change. Written by a scientist for nonscientists, this primer humanizes the great environmental issues of our timethe hole in the ozone layer, the greenhouse effect, acid rain, and air pollutionand explains everything in accessible prose. A new preface takes into account developments in environmental policy that have occurred since publication. Highlighting the interrelatedness of human activity and global change, Richard Somerville stresses the importance of an educated public in a world where the role of science is increasingly critical.
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The Forgiving AirUnderstanding Environmental Change
By Richard C. J. Somerville
University of CaliforniaCopyright © 1996 Regents of the University of California
All right reserved.
Chapter OneAir Pollution and Acid Rain
We move now into areas of more direct local effect-air pollution and acid rain. One difference between these topics and the global effects of ozone depletion and greenhouse warming is that ordinary meteorology is crucial to many aspects of local pollution. Until now, we've managed to discuss global issues without getting into the basics of weather, without talking about fronts and storms and wind and rain. To understand air pollution, we must become a little more informed about what makes daily weather.
In simplest terms, if you analyze a pollution problem like Los Angeles smog, which is a classic urban air-pollution problem, you can think of it as a process that begins with sources of emissions. The most important source of Los Angeles smog is automobile exhaust, but of course there are other important sources, too. Each source emits its own mix of pollutants into the atmosphere, and the atmosphere is like a great big pot that contains this evil stew. It allows the pollutants to mix together and interact with one another. It exposes them to sunlight and transports them from place to place. And along the way it allows various changes to occur. The result is that pollution, that atmospheric stew, influences "receptors" like your lungs, the paint on your car, and crops in the field.
What goes on in the atmosphere is absolutely crucial to the situation. But so is location. If Los Angeles weren't where it is and if it didn't have the kind of weather regimes it has, it wouldn't have the air pollution it has, even though the main cause is people driving cars.
Health effects, an aspect of air pollution I'm not going to say very much about, is in some ways the most crucial. Although a great deal is known about the effects of air pollution on human health, like much medical knowledge our understanding is still incomplete in biophysical terms. We don't know with any certainty what happens to someone's lungs when sulfate particles invade them. It's true that air pollution can increase the chance of bronchitis and emphysema and so on, and there is a great deal of statistical evidence to support such claims. Smog alerts, in short, are a good idea. But the deeper you delve into the subject, the more you realize how incompletely understood it is at a fundamental level.
Here we'll concentrate on the meteorology and chemistry of air pollution. We begin with some aspects of meteorology. Let's consider circulation in the atmosphere, first in global terms, because it's relevant to what happens in Los Angeles.
What drives the large-scale circulation of the atmosphere is heat from the Sun. In the tropics, near the Equator, a given area of the surface of the Earth receives much more solar radiation on average than does an equivalent area at high latitudes, near the poles. Therefore, the warmest places on Earth are in the tropics. Especially over the tropical oceans, this warmth leads to convection in the atmosphere: the air closest to the surface absorbs heat from the surface, which is warmed by sunlight, and as that air is warmed, it becomes buoyant, and rises. A visible manifestation of that process, noticeable if you are in the tropics lying on the beach or flying over the area in a satellite, is impressive towers of convective clouds, which are typical, for example, of thunderstorms: tall, puffy white clouds, cumulus and cumulonimbus clouds. The presence of convective clouds is a sign of rising air. The rising air in this convection transports moisture and heat to higher altitudes.
The air that rises in the tropics has to come down somewhere. Thus, a circulation is established: the air rises near the Equator, flows poleward (northward in the Northern Hemisphere, southward in the Southern Hemisphere), descends in the subtropics (latitudes between about 20° and 30°), and returns to the Equator near the surface. This circulation, which resembles a gigantic sea breeze, is called the Hadley cell, after a scientist who studied it long ago.
This circulation is a statistical construct. What do we mean by that? If you go out one fine day to some point in the Northern Hemisphere tropics, say Martinique, and take measurements, you will not necessarily find that the wind that day blows from north to south (toward the Equator) near the surface and from south to north (toward the North Pole) at higher altitudes. But if you take many measurements, not just in Martinique but at many tropical locations, and you average them over days and weeks and months, you'll find that on average the circulation behaves in the sense implied by the Hadley cell, the motions being upward at the Equator, poleward aloft, downward in the subtropics, and toward the Equator near the surface. The statistical, long-term, average property of the atmosphere is such that in the tropics the large-scale north-south circulation is a Hadley cell, circling the globe.
A typical latitude of this descending motion in the subtropics is in the range from 20° to 30°. The reason why it is often sunny in these latitudes is that the descending motion of the Hadley cell suppresses atmospheric convection. Clouds generally require upward motion for their formation, and it is more difficult for a cloud to form if the large-scale vertical motion of the atmosphere is downward. Thus, the absence of convective clouds is a typical sign of descending air motions. All the great deserts of the world are in the subtropical latitudes. It's not an accident that the desert Southwest in the United States is about at this latitude rather than at, say, 45° north. It's no accident that the Sahara Desert is mainly in the 20°-30° north-latitude belt. The great Australian desert and Chile's bone-dry Atacama Desert are in the corresponding latitude belt in the Southern Hemisphere.
Though there are many variations from one longitude to another-not every place in the 20° to 30° latitude belt has a sunny, dry climate-on average, over the whole Earth, over many years, the basic feature of the large-scale, global-average, north-south circulation of the tropical atmosphere is the Hadley cell.
Why doesn't the Hadley cell extend all the way to the poles? Why don't we have a simple picture of rising motion in the tropics, with air flowing toward the poles at some altitude, descending near the poles, and returning toward the Equator close to the surface? Why does the Hadley cell extend only to about 30°? That's a profound question, the answer to which has been debated for centuries. Many wrong answers have been given by many thoughtful people. We know now that the answer is not simple. There is no relatively short statement you can make that shows exactly why the Earth causes this circulation to be what we observe it to be, rather than what we might think it ought to be.
We do know that the answer has to do with the fact that the Earth rotates: if the Earth didn't rotate, or if it rotated much more slowly, the circulation might well descend near the poles, instead of near 30°. The answer also has to do with other quantitative aspects: how far the Earth is from the Sun, and therefore how much energy it receives; how much moisture we have in the atmosphere; and how likely it is that water will change from one phase into another (solid, liquid, and gaseous water all occur in the atmosphere). The main factors causing the circulation of the Earth to be what it is are the temperature contrast between the poles and the Equator, and the rotation of the Earth. If the Earth rotated at a very different rate, there could be a very different circulation.
We do know that the circulations on other planets are markedly different from the Earth's circulation. This is a fascinating topic, one that really has come into its own only since the advent of space exploration. Analyzing the atmospheres of other planets makes us realize how different the circulations can be. In fact, no other planet in the solar system has anything like the same meteorology as the Earth. Why these things are as they are is another of those questions to which there is no simple answer. Rotation and heating are the critical elements. In these two respects, the planets all differ greatly from one another.
What we observe, first of all, is that the air that flows back near the surface of the Earth from the subtropics toward the Equator doesn't flow directly from north to south. Instead, it gets turned to the right (in the Northern Hemisphere) by a deflecting effect due to the Earth's rotation. This effect, called the Coriolis effect, arises because the rotation of the Earth produces an apparent acceleration that doesn't change the speed of the wind, but does affect its direction. The Coriolis effect needs to be taken into account in understanding many large-scale motions on the Earth, not just the meteorological patterns. For example, to calculate accurately how to launch a projectile, such as an artillery shell, from one location to another one far away, it's necessary to take into account the fact that, while the projectile is in flight, the target will appear to have moved because of the rotation of the Earth. Allowing for the Coriolis effect is unnecessary for rapid, short motions, but it's essential to slow, long ones, such as the large-scale winds.
North of about latitude 30° in the Northern Hemisphere, and south of about latitude 30° in the Southern Hemisphere, we have the belt that includes most of the United States, sometimes miscalled the temperate latitudes (it's often not very temperate there). These are the latitudes where we find the great migrating storms that characterize the weather of many parts of the United States. It's an area of complex atmospheric flow where the big highs and lows on the weather map migrate slowly from west to east.
The tracks of these storms often tend to follow the jet stream, an intense west-to-east band of rapidly moving air. The jet stream is found not near the surface, but at the top of the troposphere. It's the reason why it routinely takes less time to fly from the West Coast to the East Coast than it takes to fly back to the West Coast.
The jet stream can be thought of as resembling a river. In the river are eddies, and those eddies are associated with the storms, the highs and lows moving across the weather map. The storm systems tend to follow the jet stream. For a long time meteorologists called it the steering current, because it allows you to predict where today's cyclone will be tomorrow. On average, the jet stream appears as a narrow ribbon of fast-moving air, but in fact it meanders; its position changes a lot from day to day.
The weather in some areas is especially susceptible to variations in the jet stream. California is one. Often, whether a particular winter in California is a wet season or a dry season depends on the position of the jet stream. Southern California usually doesn't get very much rain. Most of the rain that does fall there comes from a few intense winter storms. There are winters when all the storms seem to go north, leaving Southern California dry. Sometimes you'll hear that called a blocking pattern. That means that a region of high pressure diverts the jet stream to the north. Why this happens is complicated, but that's a typical circulation during dry California winters.
The concept of fronts is more straightforward. Imagine that cold air is coming down from the north. As the cold air mass advances, it displaces warmer air. A front is just a boundary between air at one temperature and air at another temperature. So if this cold front is moving southward and you're in its path, then as time goes on the front will cross over you, and instead of being in the warm air, you'll be left in the cold. That's why it's called a cold front. Similarly, if the movement of the air mass is in the opposite direction, so that warm air is advancing, that's a warm front. If you're standing still and are initially in cold air, then after a warm front passes you, you'll find yourself in warm air.
In winter the temperature contrast between the Equator and the high latitudes, between the tropics and the polar regions, is strongest. For although the temperature in the tropics doesn't change very much all year, it's of course coldest in high latitudes in winter. It is that temperature difference between low latitudes and high latitudes that drives the entire atmospheric circulation. The Hadley cell, for example, is much more intense in winter than it is in summer. Storms in middle latitudes are, in general, also more intense in winter than in summer. These storms derive their energy from the temperature difference between high latitudes and low latitudes.
Land masses and mountain ranges have a powerful effect on all these weather patterns. We've been talking about the average conditions over the whole Earth, but it's clear that where you're located-whether, for example, you're on the west side or the east side of a continent-greatly affects your local weather. Storms generated in regions of great temperature contrast tend to form off the east coast of the continents. Air in middle latitudes usually moves from west to east, and when an air mass remains over a continent for a long time-in winter, when the continents are colder than the oceans-the air mass gets cold. Then, when the air moves eastward and hits the relatively warm ocean, it presents a strong temperature contrast. That change in temperature can produce an intense storm. That's why such storms form off the east coast of the United States in the region of the Gulf Stream and off the east coast of Japan and Asia in the region of the Kuroshio current. These are the regions where the greatest heat transfer from the ocean to the atmosphere occurs, and the regions where many of the storms in the middle latitudes tend to originate. The effects of mountains, like the effects of continents and oceans, are important as well.
The part of the Hadley cell that flows toward the Equator in the tropics at low altitudes is called the trade winds. In general, trade winds blow from east to west. By convention, we call a wind blowing from east to west an easterly wind. An easterly comes from the east; it doesn't go toward the east. So trade winds are easterlies. Because of the Coriolis effect, trade winds blow generally from northeast to southwest in the Northern Hemisphere and from southeast to northwest in the Southern Hemisphere, converging near the Equator.
The modern name for this region near the Equator where the trade winds from both hemispheres converge is the Intertropical Convergence Zone (ITCZ). You can see it very clearly in satellite photographs.
Excerpted from The Forgiving Air by Richard C. J. Somerville Copyright © 1996 by Regents of the University of California . Excerpted by permission.
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Meet the Author
Richard C. J. Somerville is Professor of Meteorology at Scripps
Institution of Oceanography, University of California, San Diego. He is also a Fellow of the American Association for the Advancement of Science.
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