ATMOSPHERE

Man is able to directly utilize only a small portion of the energy of the Earth's atmosphere. Indeed, excessive concentrated energy in the atmosphere—hurricanes and tornadoes—represents risks to mankind. Most human demands created by atmospheric conditions involve consumption of energy to maintain comfort. The ambient surface air temperature, for instance, determines how much energy is needed for heating or cooling demands and the level of outside ambient illumination determines the need for artificial lighting. Electricity derived from fossil fuels powers the industrialized world Petroleum products directly power most forms of transportation. Pollution emitted by massive fossil fuel consumption affects man's well-being and quality of life on both immediate and long-term time scales, and mitigation this anthropogenic (manmade) pollution using emission-control devices requires even greater energy consumption.

ATMOSPHERIC COMPOSITION

Unpolluted air contains about 78 percent molecular nitrogen, 21 percent molecular oxygen, 1 percent argon, up to 3 percent water vapor, and a host of trace gases, including carbon dioxide, carbon monoxide, methane, nitrous oxide, helium, krypton, and radon. Oxygen is constantly released to the atmosphere by green plants during photosynthesis. Plants and animals excrete carbon dioxide during respiration. Water evaporates from the surface of the Earth and travels as a vapor through the atmosphere, eventually condensing and falling as precipitation. The atmosphere-ocean-geosphere and biosphere have maintained a natural chemical balance over many millennia, although steadily increasing anthropogenic trace gas emissions may have the potential to change this natural balance in the future.

Air pollution is produced by various natural and anthropogenic sources. Natural sources inject large amounts of particles into the atmosphere, including inorganic minerals, pollen, small seeds, bacteria, fungi, and effluvia from animals, especially insect parts. These natural particles usually have diameters greater than 10^-5 cm. Many anthropogenic particles are continuously injected into the atmosphere, including latex and soot. Particles produced from combustion generally have diameters smaller than 10^-5 cm. Tiny hygroscopic particulates from both natural and anthropogenic sources play an important role in the atmosphere, serving as condensation nuclei for water droplet and ice crystal formation. The period of time that particles remain in the atmosphere is influenced by their height and weight. Small particles in the stratosphere remain aloft much longer than small particles in the lower troposphere.

Polluted air often contains carbon monoxide and volatile organic carbon (VOC) gases, including ketones and aldehydes, as well as oxides of sulfur and oxides of nitrogen. Anthropogenic emissions of these gases arise from incomplete combustion and subsequent photochemical alterations in the atmosphere. Anthropogenic emissions also inject a number of relatively inert gases into the troposphere, including chlorofluorocarbons, sulfur hexafluoride, and carbon tetrachloride. Trees have been found to be a major natural source for VOCs.

Pollutants have various atmospheric residence times, with reactive gases and large aerosols being rapidly removed from air. In the London air pollution episode of December 1952, the residence time for sulfur dioxide was estimated to be five hours; daily emissions of an estimated 2,000 tons of sulfur dioxide were balanced by scavenging by fog droplets, which were rapidly deposited. Most relatively inert gases remain in the atmosphere for extended periods. Sulfur hexafluoride, used extensively in the electric power industry as an insulator in power breakers because of its inertness, has an estimated atmospheric lifetime of 3,200 years.

Emissions from fossil fuel combustion have caused increasing air pollution problems. Four major types of problems have been recognized: acid deposition and acid rain, air pollution episodes involving sulfur-rich smog from coal burning, photochemical smogs from gasoline-powered vehicles, and the threat of global warming as a result of increasing levels of carbon dioxide (a "greenhouse gas") in the atmosphere.

North American and Western European countries responded to acid rain, acid deposition, and acidic sulfurous smog episodes (which caused excess mortality and morbidity as well as greatly decreased visibility) by passing emission control laws. Sulfurous smogs are now rare in North America and Western Europe. Despite emission controls, acid rain and acid deposition are believed by many scientists to be the cause of forest decline, known as "neuartige Waldschäden" in Europe. This forest decline has been detected throughout Central Europe at all elevations and on all soil types. Evidence suggests that nitrates in acid rain play an important role in Waldschäden. Asian countries continue to burn fuel with few emission controls. A few tragic consequences include erosion of the Taj Mahal by acidic air pollutants, and occasional snoot-laden smog blanketing the Indian Ocean north of the equator.

Photochemical smogs arise worldwide because of the action of sunlight on emissions from gasoline-powered vehicles. Decreased visibility, increased morbidity, and crop damage as a result of photochemical smogs led to introduction of the catalytic converter on automobiles in the United States. This has had only a small impact on the occurrence of photochemical smogs in the United States.

Global warming has attracted growing worldwide concern, leading to the Kyoto Accords of 1997, which agreed that rich industrial nations would reduce greenhouse gas emissions. Legally binding reductions for each major greenhouse gas were set, with emphasis on reducing carbon dioxide emissions by 2008–2012. The Kyoto Accords were signed by President Clinton in 1998, with a carbon dioxide emissions reduction objective of 7 percent for the United States, although the U.S. Senate failed to ratify them. China, Brazil, India, and Mexico were among nations exempted from the Kyoto Accords. Canada (with an emissions reduction objective of 6%) and the European Union (with an emissions reduction objective of 8%) have developed (and implemented) some strategies to reduce carbon dioxide emissions. Norway has begun a program to sequester carbon in the ocean.

The concentration of chlorofluorocarbons in the atmosphere has been steadily increasing since they began being manufactured. It has been discovered that chlorofluorocarbons are slowly destroyed by chemical reactions in the stratosphere, especially heterogeneous reactions in polar stratospheric clouds above Antarctica. The chlorine released during these reactions in turn destroys stratospheric ozone, the most prominent result being the creation of the infamous "ozone hole," a zone with greatly diminished stratospheric ozone centered over Antarctica during winter. This ozone depletion occurs in the winter and early spring—when the sun's radiation strikes the Antarctic stratosphere. Ozone levels recover

Recognition of the threat of stratospheric ozone depletion posed by chlorofluorocarbons and chlorofluorohydrocarbons led 131 countries to sign the Montreal Protocol in 1987. Production of chlorofluorocarbons was banned as of January 1, 1996, because of their potential to further deplete stratospheric ozone. Chlorofluorohydrocarbons will be phased out of production by 2030; HCFC-22 will be phased out by 2020. However, large amounts of chlorofluorocarbon refrigerants produced over many decades remain in use worldwide, awaiting future release.

EARTH'S RADIATION BALANCE

Solar radiation continually reaches the earth, warming the atmosphere, ocean, and land surfaces on the sunlit portions of the planet. Although the sun emits a continuous spectrum of electromagnetic energy, its peak emissions are in the visible wavelengths, with a maximum at 500 nm wavelength. The average amount of solar energy received globally at the top of the atmosphere is relatively constant, about 1,353 W/m². Clouds and particles reflect some incident solar radiation back into space. Some large volcanic eruptions inject copious numbers of particles, which attenuate solar radiation reaching Earth's surface. When a volcanic eruption injects large amounts of sulfur into the stratosphere, sulfuric acid aerosols slowly form in the stratosphere, where they remain for months; these aerosols also reflect incident solar radiation.

About 51 percent of solar energy incident at the top of the atmosphere reaches Earth's surface. Energetic solar ultraviolet radiation affects the chemistry of the atmosphere, especially the stratosphere where, through a series of photochemical reactions, it is responsible for the creation of ozone (O₃). Ozone in the stratosphere absorbs most of the short-wave solar ultraviolet (UV) radiation, and some long-wave infrared radiation. Water vapor and carbon dioxide in the troposphere also absorb infrared radiation.

Considerable energy is radiated back from Earth's surface into space as long-wave infrared radiation. The atmosphere absorbs some of this infrared radiation, preventing its loss to space. This trapping is sometimes referred to as "the Greenhouse Effect."

THE HYDROLOGICAL CYCLE

Water is constantly evaporated from rivers, lakes, and oceans, and released from vegetation through evapotranspiration. Water vapor travels through the atmosphere, eventually forming small droplets or ice crystals in clouds. Some particles grow sufficiently large, and fall as rain or snow. Most precipitation occurs over the world's oceans. Much of the rain and snow falling over continental areas rapidly runs off into major river channels, returning water to the oceans. Some snow is deposited in glaciated areas, including high mountain peaks on the continents and on the Greenland and Antarctic ice sheets, where it may remain for millennia. About 75 percent of the Earth's fresh water is currently stored in glaciers ice sheets. Calving of icebergs from the ice sheets and periodic glacial retreat in major mountain ranges return some of this long-frozen water to the ocean. During the summer of 1999, no icebergs were seen to enter North Atlantic shipping lanes, possibly because of warmer than usual ocean temperatures.

The balance between evaporation, precipitation, glaciers, and oceans, known as the hydrological cycle, is usually considered to be in rough equilibrium over the Earth, although there is evidence that the Greenland ice sheet shrank substantially during the mid-1990s. There is also evidence that the West Antarctic ice sheet thinned during the same period.

During times of global cooling and glacial advance, ocean water levels dropped as increasing amounts of water were stored in ice sheets, as was the case during the Pleistocene glaciation of Eurasia and America. At warmer times in the geological record, glaciers have had dramatic retreats, resulting in a worldwide rise in ocean levels. The change from a cold period to a warm period may occur rapidly over the course of a century. Since 1960, mid-latitude glaciers have receded dramatically. Glaciers in the Caucasus are estimated to have lost about half their mass, while the Tien Shan Mountains are estimated to have lost about 22 percent of their glacial ice between 1960 and 2000. The decade of the 1990s is believed by most scientists to have been the warmest in many millennia. It has been postulated that these changes foreshadow a more prolonged global warming period that may be partially attributable to anthropogenic alterations in atmospheric composition.

TROPICAL STORMS

Solar heating of tropical oceans warms the surface water, promoting evaporation. Where the equatorial surface waters are warmest and the northeast and southeast trade winds meet, a band of cirrostratus and cirrus clouds spreads out from convective precipitation regions. This area is known as the Intertropical Convergence Zone.

When tropical ocean surface water temperatures exceed 26°C near the edge of the Intertropical Convergence Zone, and air aloft is warm and moist, conditions are favorable for the development of large tropical cyclones. These storms begin as weak depressions or disturbances, most of which fail to develop into organized systems. When conditions favor storm development, pressures drop in the center and winds increase in a tight 30–60 km band around a central eye. Large storms are powered by the latent heat of condensation released as clouds form from the moisture-laden air.

When a tropical storm has winds in excess of 120 km/hr, it is officially classed as a hurricane. Large hurricanes have a highly organized rotary structure, with a central eye surrounded by tightly curving bands of clouds extending up to 2,000 km in diameter, although most important activity occurs within 100 km of the eye. Large hurricanes draw enormous amounts of moisture from the Intertropical Convergence Zone.

The most powerful hurricanes (called "Category 5") have sustained winds exceeding 248 km/hr. In general, hurricanes move slowly with the average wind speed of the troposphere. When these hurricanes strike land, they bring a devastating combination of high winds, torrential rain, and a storm surge. The storm surge is an uplifting of the water level resulting from an air pressure drop and wind-driven water; the most powerful hurricanes have a storm surge exceeding 18 feet (5.5 m). Hurricane Gilbert, a massive Category 5 hurricane in 1988, dominated about 20 percent of the entire global Intertropical Convergence Zone, causing the cloudiness in the zone outside the storm to dissipate. Hurricane Andrew, which devastated South Florida in 1992, was also a Category 5 hurricane.

As a hurricane travels over warm ocean water, it lowers the sea surface temperature by about 3°C in a 100 km swath. When a hurricane is stationary, this surface ocean cooling weakens the storm intensity. Hurricanes also rapidly lose strength when they move over cold water or land.

How is energy utilized in a hurricane? Hurricanes derive energy mainly from the release of latent heat, and lose energy, in part, through precipitation and frictional loss of the wind. For an average hurricane, the rate of release of latent heat has been estimated at 10¹⁴ watts. This is equivalent to the energy output of a 20-megaton bomb every fifteen minutes. An average hurricane with maximum winds of 50 m/s and a radius of 30 km dissipates wind energy at a rate of 3 × 10¹² watts. Thus, it takes only about 3 percent of the input energy to maintain the destructive winds of an average hurricane.

THUNDERSTORMS

Thunderstorms (cumulonimbus clouds) come in many sizes and shapes, ranging from small "air-mass" thunderstorms to large "supercells." Thunderstorms are influenced by the surrounding atmosphere and nearby convective activity. Sometimes a thunderstorm is composed of a single, isolated cumulonimbus cloud. At other times, cumulonimbus clouds are so numerous that they form a continuous sheet, losing any separate identity.

The air-mass thunderstorm is the least severe of all thunderstorms. In its simplest form, an air-mass thunderstorm grows as a single cell when solar radiation heats the surface air in an unstable atmosphere. Its life cycle lasts around 30 minutes. Towering cumulus clouds are formed as in-cloud updrafts push moisture upward. The tower may reach a height about five times the diameter of the cloud base in the growth phase.

When water vapor is deep enough for continued convective activity, the thunderstorm reaches an active phase, in which the top of the cloud glaciates, often forming a distinctive anvil. Strong updraft and downdraft regions form within the cumulonimbus cloud. The change from a towering cumulus cloud to a cumulonimbus cloud is usually quite rapid as the top turns to ice, and lightning and heavy rain begin.

The final stage in the life of a cumulonimbus cloud is marked by dissipation. The lower regions of the cloud break up, while the upper anvil spreads out. Mixing with the environment lowers vertical wind velocities by reducing the in-cloud temperatures through evaporation and mechanical mixing with the cooler surrounding air.

Most air mass thunderstorms form in groups, facilitating growth by the reduction of environmental mixing. These multicellular storms may occur as compact clusters of cells or, if there is some external organization, laterally aligned in squall lines.

Supercells have greater size, organization, and duration than air mass thunderstorms. A supercell rotates, with persistent updrafts and downdrafts, and lasts for many hours. Updrafts in supercells may exceed 140 km/hr. Supercells develop when there are large changes in wind velocity with height. Moist, warm air entering from the front side is lifted at the cold air gust front until condensation occurs, releasing latent energy. This air parcel then moves rapidly upward and, usually, out ahead of the storm at upper levels in the atmosphere. Dry air moves in from the back side of the supercell, is cooled by rain falling out of the rising air, and then descends as a downdraft. Several different arrangements of this flow are possible. Supercells frequently move slower than the mean winds aloft. These storms are notorious for their ability to spawn tornadoes; they may show a tornadic "hook echo" on radar displays. Supercells and regular cells can combine in a multicellular complex, which then exhibits some characteristics of both types of storms.

Thunderstorms arise from convective activity, driven by energy derived from the latent heat of condensation and sublimation of water vapor within cumuliform clouds. Buoyant air movements caused by surface heating, by orographic (relating to mountains) forcing, and by lifting of warm moist surface air along frontal zones, are some of the important mechanisms for initiating the upward transfer of energy.

TORNADOES

Calculations of tornado energy are difficult to make—the aftermath of a large destructive tornado sometimes resembles carpet bombing in a war situation, with buildings ripped off foundations, large numbers of trees uprooted, and asphalt stripped from roadways. Several reports describe the derailment of up to five train cars as tornadoes have apparently lifted cars off the tracks. Large building debris has been found at a distance of 20 km from its original location.

Meteorologists categorize tornadoes by their wind speeds as deduced subjectively from severity of the damage. Each tornado is given a Fujita F-scale class: F0 (light damage), 40–72 mph; F1 (moderate damage), 73–112 mph; F2 (considerable damage), 113–157 mph; F3 (severe damage), 158–206 mph; F4 (devastating damage), 207–260 mph; F5 (incredible damage), 261–318 mph. The highest reported toronto wind speed (reported from Doppler reader) was 318 mph in the F5 Oklahoma City tornado of May 3, 1999.

The F-scale classification is only a first approximation to tornado damage. Some buildings are wind-sensitive while others are wind-resistant. The lower pressure of the tornado core also weakens the integrity of the building. Mobile homes, wood-frame houses, buildings with sheet metal roofs, and those with unreinforced masonry walls are particularly sensitive, often damaged by winds less than 100 mph. In rural counties without building codes, wood-frame houses using nails to anchor walls to foundations can be blown off and destroyed by 80-mph winds. Structurally engineered buildings are seldom destroyed or even severely damaged. People are most often injured and killed by falling building materials and by projectiles in the debris suspended in the tornado.

Tornadoes form in several ways. The most common tornadoes form at the edge of thunderstorm cold air outflow, and they are called gustnadoes. Gustnadoes fall into the F0 or F1 class and only rarely inflict intense damage along a short, narrow path. Waterspouts and landspouts form in areas where a pre-existing surface circulation becomes entrained, stretched, and intensified as a thunderstorm updraft passes over. Waterspouts and landspouts may attain in F2 class, and several have been reported to inflict moderate damage to marinas or poorly constructed buiildings. The least frequent and most severe tornadoes form and descend from supercell thunderstorms, which may persist for many hours and spawn multiple tornadoes. However, the most severe tornadoes, although less frequent, are those that descend from supercells. Supercells may persist for many hours. A single supercell, moving over several hundred kilometers, has been observed to spawn a series or "family" of up to eight tornadoes along its route. These tornadoes are associated with rotating circulations called mesocyclones. The mesocyclone is 10 to 20 km in diameter, much bigger than a single tornado. It can sometimes be seen as a larger-scale rotation of the clouds at the bottom of the supercell. Rotation begins at an altitude between 4 km and 8 km and builds downward. Sometimes a mesocyclone produces more than one tornado at a time.

An average of 800 tornadoes is reported within the United States yearly, with possibly 2,000 small tornadoes going unreported. Tornadoes have been reported in every state, including Hawaii and Alaska. The Great Plains has the highest occurrence of damaging tornadoes. Occasionally, tornadoes occur in outbreaks. The super-outbreak of April 2 and 4, 1974, had 148 reported tornadoes in 13 states. Hurricanes can spawn tornadoes; in 1967, Hurricane Beulah generated 115 reported tornadoes. The majority of tornadoes occur in late afternoon and evening. However, tornadoes can form at any time of day or night. Nocturnal tornadoes are relatively common on the U.S. Gulf Coast.

Television news reporting gives the impression that people incur substantial risk in tornado-prone regions, but the likelihood of any particular building being hit is on the order of once every million years.

LIGHTNING

Within cumulonimbus clouds, precipitation processes and ambient physical conditions interact to produce regions of high electrical charge. The mechanisms by which charge separation occurs in cumulonimbus clouds are poorly understood by cloud physicists. Some researchers believe that electrical charges build in strength when ice pellets fall through a region of ice crystals and water droplets.

Lightning is the visible manifestation of a plasma channel. The plasma is very hot, with peak temperatures greater than 30,000°C, compared to 6,000°C for the sun. Although the peak current in a lightning stroke may be as high as 100 kiloamperes, charge transfer is limited by the brief duration of the flash. Movement within the plasma is limited; a typical electron in the lightning channel may move only two meters. Most of the charge transfer occurs by way of a continuing current between the strokes comprising the flash, and by relatively low amplitude currents following strokes. Usually lightning transfers negative charge to the ground. However, positively charged cloud-to-ground lightning also occurs.

After electrical potentials on the order of 300 to 400 kV/m are produced in discrete regions within the cloud, streamers extend their way forward along the cloud's charge gradient in a tree-like structure. When electrical potentials on the order of 1,000 kV/m develop, streamers become self-propagating. A plasma channel then moves toward regions of opposite charge within the cloud, neutralizing much of the electric charge within the cloud as it travels through diffusely charged regions. As the channel tip advances, it may branch in several directions simultaneously. If it penetrates into highly charged a region, a recoil streamer may flow along the channel to the initiating region.

About 80 percent of lightning channels begin and end in the cloud. The remaining 20 percent of streamers extend horizontally into the clear air outside the cloud. They propagate in a stepwise fashion called step leaders. Discharges ending in the clear air are usually highly branched, and generally quite weak. When a step leader approaches the earth's surface, an upward streamer propagates from the ground toward the channel tip aloft. These plasma channels intersect at an altitude of about 100 m above the ground. Completion of the circuit causes an upward rush of electrons called a return stroke, substantially increasing the brightness of the luminous plasma channel. Frequently, a second pulse of energy, the dart leader, moves smoothly down from the cloud, following the same path to the ground. Return strokes may follow the dart leader. Typically, cloud-to-ground flashes have four or more separate strokes.

Capturing electricity from a stray lightning flash is an intriguing but impractical idea. Presumably, Benjamin Franklin, in his famous kite experiment, transferred energy from a lightning flash to a Leyden jar, a primitive type of battery. A typical lightning flash has 25 coulombs of charge and 30,000 instantaneous amps. However, the stroke is very brief; 0.01 seconds. This is only enough energy to power one 100-watt bulb for a few months. One hundred thousand 1,000-ft towers would be needed to capture lightning energy equivalent to the output of a typical small power station.

Very powerful lightning discharges, known as superbolts, are about 100 times more powerful than the typical lightning stroke. Superbolts are most common in the wintertime off the coasts of Japan and the eastern United States. The radius of a superbolt channel is estimated to be 20 cm, compared to 2 cm of a typical lightning stroke. Because superbolts are thought to be rare over land, tapping energy from them is even more problematic than obtaining energy from a regular lightning strike.

High-altitude discharges above active thunderstorms have been studied with cameras that sense very low light levels. Several distinct, differently colored phenomena have been identified. Unusual cloud-to-air discharges from the anvil top upward to heights of 35 to 50 km (into the stratosphere) are called "blue jets." Blue jets propagate upward at about 100 km/sec in a narrow conical or trumpet shape. Red discharges extending 50 to 95 km upward above thunderstorm anvils are called "sprites." Sprites have widths ranging from 10 km to 50 km, and have been observed only above large (30,000 km²) multicellular thunderstorms. "Elves" are very brief (1 msec) red halos that form at altitudes of 60 to 100 km.

WINDS

Winds arise through a complex interplay of forces. As Earth rotates around its axis every 24 hours, the atmosphere moves along with the earth. In the troposphere, large-scale weather systems, covering regions of around two million square kilometers, form an interlocking grid pattern over the globe. The growth and decay of these large systems produces day-to-day changes in weather conditions around the world. Large-scale weather systems develop quickly; they may double in intensity in a period of 12 to 48 hours. Once formed, these systems decay slowly, generally halving in intensity in four days.

Temperature swings following frontal passages are common in North America and Eurasia, but are rare in the tropics, where differences in cloudiness and precipitation arise from seasonal variability in thermal forcing. Poleward from the tropical regions, extratropical cyclones transform latitudinal temperature gradients into kinetic energy.

In coastal areas, temperature differences between the land and the water produce air pressure variations, creating sea and lake breezes that are superimposed on the normal winds. These winds vary diurnally and as a function of cloudiness. During the daytime, winds blow from the cool sea toward the warm land, while at night the land becomes cooler than the sea surface, and the winds blow from land to sea.

On a larger scale, continents produce flows, known as "monsoon winds," over wide areas between the surrounding seas and lands. These winds respond to seasonal forcing. The best example is the Indian monsoon. During the summer monsoon, from June through September, moist winds blow northward off the Indian Ocean. Convergence of this moist air with other air masses produces intense precipitation. The monsoon slowly moves northward in spring and summer, traveling about 5 km/day. From December through February, the Siberian high dominates Eurasian air circulation, and the general flow of air is reversed, with cold, dry air traveling from the continental land masses southward over the warmer surface waters of the Indian Ocean.

Topography can substantially change air flow. Local mountain winds form when surface heating causes winds to flow up the sides of the mountain: technically known as "anabatic flow." Anabatic winds are generally strongest in early afternoon. At night, winds flow down off hills or mountains, technically known as "katabatic flow." In hilly terrain, with slopes of about two degrees, winds on the order of 3 km/hr descend as the ground surface cools.

Mountains modify the velocity and direction of wind. The coastal mountains along western North, Central, and South America play a major role in determining regional winds on the eastern rim of the Pacific Ocean. Dynamically induced winds may attain substantial speeds in mountainous regions, sometimes exceeding 100 km/hr. Some orographic winds have been given names associated with a specific region, such as the "Santa Ana" winds that occur as dry continental air descends from the Sierra Nevada Mountains to Southern California coastal areas during spring and autumn. A strong, warm wind on the leeward side of a mountain range is called a chinook (North America) or föhn (Europe). Strong chinooks, with damaging winds reaching 160 km/hr, occur several times each winter along the Front Range of the Rocky Mountains.

WIND ENERGY EXPLOITATION

Wind turbines produce power by converting the force of the wind into torque. The power produced is a function of the wind energy flux (power), which, in turn, is a function of the air density multiplied by the wind velocity raised to the third power. Changes of air density with time at a particular site are negligible compared to the fluctuations in wind velocity. Meteorologists usually report wind speed as an average. To get the potential wind power, the average Areas with annual average wind speeds of 13 mph or greater are found throughout the United States. Regions with Class 4 winds are considered attractive for wind turbine siting. (Developed by Battelle Pacific Northwest Laboratories for the U.S. Department of Energy) wind speed is raised to the third power and then adjusted using a Weibull statistical distribution too account for the natural instantaneous wind variability.

Wind speed, and thus available wind power, at any given location is a function of several factors: global variations; local variations, especially around coast lines with sea or lake breezes and topography; and diurnal variations of wind speed from differences in the stability of the air next to the ground. Turbulence associated with unstable air during the afternoon, or on cloudy days, mixes higher velocity winds aloft with the winds slowed by friction at the surface. On clear nights the air is stable, and there is little transport of the high winds aloft to the ground. Thus, wind speeds near the ground are normally higher during the daytime than at night, with the highest wind speeds occurring in the afternoon, and minimum wind speeds around dusk and dawn. In general, gusts are greatest in the afternoon.

Wind speed varies with height above the ground. Because surface wind speeds are routinely measured at 10 m, winds turbine heights (usually higher than 10 m) must be estimated. The turbulence level of wind also varies. Forests, buildings, and other obstacles slow wind down, and increase turbulence levels. Long grass, shrubs, and crops can slow the wind considerably. Such variations can be corrected by use of "roughness classes" or "roughness lengths." The sea is assigned a roughness class of zero, while a landscape with many trees and buildings has a class of three or four. (Sheep can keep the roughness down through grazing.) When no data are available for a site, a wind rose from the nearest observations may provide a rough estimate of wind speed. However, data availability frequently is sparse in areas with substantial wind generation potential.

During the late 1970s and early 1980s, there was considerable interest in harnessing wind energy in the United States. During this time, efforts were made to determine the national wind energy potential. Maps were drawn using the "Batelle Wind Power Classes," ranking nominal wind energy at 10 m, 30 m, and 50 m elevations. These classes, which remain standard in mapping wind energy, are shown on the map. In general, Class 4 and higher winds are considered favorable for wind energy exploitation in the United States.

Because of its large population and the tacit assumption that its varied topography would be ideal for wind power exploitation, California conducted its own program to determine wind energy potential. This study demonstrated the meteorological difficulties in characterizing wind speeds in hilly terrain. Some wind turbines were constructed in areas thought to be ideal, but which proved to be quite marginal. Three California passes were identified as among the best wind energy sites in the world, with average wind speeds in excess of 8 m/s. Tehachapi and San Gorgonio have proven successful, and the Altamount Pass wind farm has over 7,500 wind turbines in operation.

Within the United States, some areas are especially suited for wind power generation, including North and South Dakota, Minnesota, Montana, Wyoming, the Front Range of the Rocky Mountains, the Cascade Mountains, the Great Lakes shoreline, and the ridge crests and peaks of the Appalachians. Close examination of specific geographical and topographical features may help wind power planners identify suitable sites. This has proven to be the case for Buffalo Ridge, a 100-km long ridge stretching from Spirit Lake, Iowa, through southwestern Minnesota north through Lake Benton to Sica Hollow, South Dakota. It has the potential to yield 3 Terawatt hours yearly.

Dennis G. BakerAnita Baker-Blocker

See also: Acid Rain; Climatic Effects; Turbines, Wind.

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Yoshino, M. M. (1975). Climate in a Small Area. Tokyo: University of Tokyo Press.

Atmosphere

Introduction

The mixture of gases surrounding Earth to a height ofaround 300 mi (482 km) is known as the atmosphere. It is composed of two main gases and around 20 trace gases. The main gases are nitrogen and oxygen, while the trace gases include carbon dioxide, water vapor, ozone, and argon. Although the main gases have remained at a constant concentration for millions of years, the concentrations of the trace gases are liable to change.

The atmosphere is composed of four layers. The troposphere, which is the layer closest to Earth, contains most of its mass. The density of the atmosphere decreases with distance from Earth’s surface. The four layers do not mix because their temperatures are different. The atmosphere plays an important role in life on Earth. However, an increase in the levels of greenhouse gases threatens to cause widespread climate change through global warming.

Historical Background and Scientific Foundations

Earth is surrounded by a layered mixture of gases known as the atmosphere. Nitrogen makes up 78% of the atmosphere and oxygen 21%. These proportions have remained unchanged throughout human history. The remaining 1% is made up of a number of trace gases including carbon dioxide, nitrogen oxides, rare gases like argon, and ozone, an allotrope of oxygen. The composition of Earth’s atmosphere was once very different, however. At the time of Earth’s origins, around 4.5 billion years ago, the atmosphere was mainly hydrogen and helium. Over time, these light gases diffused into space. Volcanic emissions would have added carbon, nitrogen, oxygen, and sulfur to the atmosphere. Most of the atmospheric oxygen appeared later, being generated by the photosynthetic activity of blue-green bacteria and green plants.

The atmosphere extends to around 60 mi (97 km) above the surface of Earth and its different layers are known as the troposphere, the stratosphere, the mesosphere, and the thermosphere. Each one has a different composition of gases. Also, the temperature of the atmosphere changes with altitude. There are no firm boundaries between the layers, but they tend not to mix because of the temperature differences.

The troposphere is the atmospheric layer closest to the surface of Earth. It contains 80-90% of the total gas in the atmosphere, with the force of gravity accounting for its relatively high density. The temperature of the troposphere falls uniformly with altitude, reaching about -58°F (-50°C) at around 9 mi (14.5 km) above the ground in tropical regions, and at around 5 mi (8 km) in polar regions. Thereafter the temperature remains constant to around 12 mi (19 km), which is the upper limit of the troposphere. This region of constant temperature is known as the tropopause. Without its existence, water vapor would rise until the energy of the sun split it into oxygen and hydrogen, which would then escape into space. Therefore, the tropopause is essential to life. Water vapor in the tropopause falls back to the surface of Earth as rain and snow. The air in the troposphere moves ceaselessly in vertical and horizontal convection currents, distributing heat and moisture around the planet and this layer is therefore the source of weather and also climate, which is the long-term pattern of the weather.

The troposphere gives way to the stratosphere, which extends to 30 mi (48 km) above Earth’s surface. Temperatures rise with altitude in the stratosphere to around 32°F (0°C) at its upper limit. The warming occurs because of chemical reactions involving ozone, which convert solar energy to heat. Around 99% of the atmosphere’s total gas is contained in the troposphere and stratosphere together. However, the stratosphere is

WORDS TO KNOW

MESOSPHERE: The third layer of the atmosphere, extending from the stratosphere to about 50 mi (80 km) above Earth.

STRATOSPHERE: The region of Earth’s atmosphere ranging between about 9-30 mi (15-50 km) above Earth’s surface.

TROPOSPHERE: The lowest layer of Earth’s atmosphere, ranging to an altitude of about 9 mi (15 km) above Earth’s surface.

far less dense than the troposphere. It is of similar chemical composition, except for containing about 1,000 times more ozone and very little water vapor. Ozone, which contains three oxygen atoms per molecule compared to oxygen’s two, absorbs ultraviolet B radiation from the sun and thereby protects life on Earth from its harmful effects.

Above the stratosphere, temperatures decline once more, reaching around -148°F (-100°C) at an altitude of 50 mi (80 km). This layer is called the mesosphere or middle layer, and it is where meteors falling to Earth burn up because of friction with atmospheric gases. Ice clouds form from water vapor in the mesosphere and these are sometimes visible from Earth’s surface. The temperature remains constant again to an altitude of 56 mi (90 km) in an atmospheric region called the mesopause. This gives way to the outermost layer of the atmosphere that is known as the thermosphere. Here, solar energy splits molecular oxygen and nitrogen into single atoms. The thermosphere extends to around 300 mi (482 km), although there is no strict boundary dividing it from the near vacuum of outer space. Temperatur begins to rise sharply in the thermosphere to 2,192°F (1,200°C) and beyond. In the lower layer of the thermosphere, pulses of high-energy radiation can cause ionized gas to emit light. The resulting glow is known as the aurora borealis and aurora australis, or Northern or Southern Lights, which are visible from certain high latitude areas, such as northern Norway.

Impacts and Issues

The atmosphere plays a vital role in controlling the temperature of Earth’s surface. Energy from the sun is partly reflected by clouds and atmospheric gases, but about a quarter of it is absorbed by the so-called greenhouse gases, including carbon dioxide, nitrogen oxides, methane, and ozone, which warms the atmosphere. Around half of the incoming solar radiation reaches Earth’s surface, where it is either absorbed or reflected. Some of this energy is re-emitted, but at a longer wavelength that is readily absorbed by the greenhouse gases. The phenomenon is known as the greenhouse effect, because the atmosphere resembles the glass of a greenhouse that traps heat inside while transmitting sunlight. If the atmosphere was devoid of greenhouse gases, then these longer infrared wavelengths would escape, leaving Earth much colder than it is now and incapable of supporting life.

Human activities such as intensive agriculture and the burning of fossil fuels like coal have increased the concentrations of greenhouse gases in the atmosphere over the last hundred years or so. This has led to a small, but significant, increase in the temperature of Earth known as global warming. It is hard to predict what the consequences of global warming might be, but climate change and change in species’ habitats seem likely to occur. The Kyoto Protocol is an initiative to secure global co-operation in limiting emissions of greenhouse

gases into the atmosphere in an attempt to limit the damage that might be caused by global warming.

Meanwhile, the chemical action of chlorofluorocarbons (CFCs), chemicals once widely used in aerosol products, has led to a depletion in stratospheric ozone which means that more ultraviolet B radiation from the sun is reaching Earth, with potentially damaging biological effects. The Montreal Protocol of 1987 aims to protect the ozone layer in the stratosphere by limiting or banning CFC use. However, it will take many years before the burden of CFCs in the atmosphere and ozone destruction is halted.

See Also Carbon Dioxide (CO₂) Emissions; Fossil Fuel Combustion Impacts; International Environmental Law; Weather and Climate

BIBLIOGRAPHY

Books

Cunningham, W.P., and A. Cunningham.Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.

Kaufmann R., and C. Cleveland. Environmental Science. New York: McGraw-Hill International Edition, 2008.

Web Sites

PhysicalGeography.net. “Introduction to the Atmosphere.” http://www.physicalgeography.net/fundamentals/7a.html (accessed February 12, 2008).

Susan Aldridge

Atmosphere

Rap duo

As co-owners and lead figures of the Minneapolis-based hip-hop label Rhymesayers Entertainment, the indie hip-hop group Atmosphere have formed a style that is uniquely Midwest and completely contrary to the MTV-friendly hip-hop of guns, bling, etc. With the duo of Slug on the microphone and producer Ant making the beats, Atmosphere has been courted by the major labels, but in alliance to their music and themselves, they have stayed on their own small but growing label. Slug's rhymes have often been emotional and personal, and Atmosphere has been written about more in indie rock magazines than in ones that cater to rap, which has little to do with the color of their skin. "Slug has a way of drawing out the universal in the intensely personal, and his gaze into the mirror reveals two faces: his and ours," wrote Christopher Bahn of The A.V. Club. Atmosphere has been labeled emo-rap, underground, or indie hip-hop, with its self-deprecating lyrics. Ten years into the group, Atmosphere has continued to break new musical ground with each release. "One can feel Atmosphere loosening modern hip-hop from its moorings and yanking it into some weirder and far more interesting place," wrote Rolling Stone's Pat Blashill, in a review of the group's 2003 album Seven's Travels.

Growing up in Minneapolis, Sean Daley was always a bit different from the other white kids in his neighborhood. When he was a teen, his parents (his father was African American and his mother was white) divorced, and Daley immersed himself in the graffiti culture of break dancing and hip-hop music. Daley started off break dancing, but discovered he was better at drawing and graffiti. He had to try his hand at everything, and after dancing and graffiti came spinning records. Emerging as a talented DJ, Daley dubbed himself Slug, and with his high school friends Stress (Siddiq Ali) and Spawn (Derek Turner), they formed The Rhyme Sayers Collective. Early live performances by the group had Slug on vinyl, making beats while Spawn MC'd. Slug and Spawn began working with other likeminded musicians who were making the kind of underground hip-hop that Midwesterners could relate to. One such peer was producer Ant (Anthony Davis). In 1998 Spawn and Slug rhymed on a record produced and made by Ant. They dubbed themselves Atmosphere and released their debut, Overcast!, on a label they co-owned, called Rhymesayers Entertainment.

Atmosphere began playing live shows around Minneapolis and the Midwest, and by 2000 Spawn had the group down to a duo. They started the Sad Clown EP series, and that year they released (now out of print) Sad Clown Dub II. A handful of singles and EPs were released as the band toured around the United States, and in 2001 they compiled 3 EPs and released them as one album. In 2001, in a distribution deal with Fat Beats, Atmosphere released Lucy Ford: The Atmosphere EPs on Rhymesayers. Village Voice writer Christian Hoard called Slug "the most openhearted MC in history." All of the songs from the EPs were written about Slug's ex-girlfriend and his broken relationship. "Lucy Ford served up an everyman persona equal parts lovelorn poet, peripatetic slacker, drunken bar regular, and class clown," wrote the Voice's Michaelangelo Matos.

Because Ant often did not tour with Atmosphere, and Slug was the front man, the rapper started getting more attention than the group, most of it based on his bare-boned emotional rhymes. In 2002 Atmosphere released their breakthrough album God Loves Ugly. The album, via distribution with Fat Beats, went on to sell more than 130,000 copies in the United States. Matos wrote that the album "feels like hip-hop: the brusque party cuts, the embattled puffed-up defensiveness, the slightly stagy sense that Slug's soul-baring tendencies have taken on now that he's gotten our attention without having to fight quite so hard for it."

Atmosphere, with a full live band, toured across the globe to promote God Loves Ugly. More than a handful of major labels tried to entice Atmosphere to join their rosters, but the group wanted to stay true to their roots, and continued to build their own Rhymesayers community. As for the title of the record, God Loves Ugly, Slug felt it was up for each listener's interpretations. "To me, it was just a basic, broad statement," he told Synthesis writer Max Sidman.

Rhymesayers signed a new distribution deal with punk label Epitaph for their 2003 release Seven's Travels. The record was hailed in popular music magazines and newspapers as heralding Atmosphere's distinctive style, and it sold more than 150,000 copies in the United States. Blashill wrote that the group made "overeducated nerd rap: self loathing, navel-gazing and occasionally hilarious," and added that "the grooves are dusty and tasteful, and Slug's words are those of a smart guy who's tired of being nice." Ant's production of jazzy and old R&B samples didn't go unnoticed either. "His dusty grooves are hooky and R&B-informed, and even when they back up Slug's most manically depressed rhymes, they never feel heavy handed," wrote Hoard, of Ant's contribution.

Front man Slug gained a lot of attention for using rhymes that seemed heartbreakingly autobiographical. Like most songwriters, though, Slug wanted listeners to know that not everything was a personal diary entry, and that his lyrics were up for personal interpretation. Slug admitted to The A.V. Club, "I grew up on Slick Rick, who could tell any story he wanted, and you never stopped and wondered if that really happened. And rap has turned into such a literal thing. … Kids actually think that rappers do these things. It's like, I got news for you, Lloyd Banks has never shot anybody, and I've never done heroin. But at the same time, I'm not going to change my technique because I'm worried about whether people are interpreting it right or wrong."

In 2005 Atmosphere issued You Can't Imagine How Much Fun We're Having, which debuted at number one on the Billboard Indie Chart with 19,000 copies sold in its first week. Performances on Late Night With Conan O'Brien and Jimmy Kimmel Live widened the group's audience to thousands of urban and suburban teens. Throughout You Can't Imagine, Slug raps about politics, murders, rage, and personal problems. "This all could have been a drag in the hands of a rapper with less self-awareness or sense of humor, or without access to the deft production skills of Atmosphere beatmaster Ant," wrote Christopher Bahn in The A.V. Club.

The Sad Clown series that the group began several years back kept Atmosphere busy. In July of 2007 it was Sad Clown Bad Summer, in November, it was Sad Clown Bad Fall. In December of that year, via free download on their Web site only, Atmosphere put up a new record aptly titled Strictly Leakage. In April of 2008, Atmosphere released the much-anticipated album When Life Gives You Lemons, You Paint That S--t Gold. The group embarked on a tour with their backup band that included Erick Anderson (keyboards), Nate Collins (guitar), Brett Johnson (bass), and Brian McLeod (drums).

For the Record …

Members include Sean Daley (a.k.a. Slug ), vocals; Anthony Davis (a.k.a. Ant ), producer. Former members include Derek Turner (a.k.a. Spawn ), vocals.

Group formed in Minneapolis, MN, c. 1998; became co-owners of record label Rhymesayers Entertainment; released debut album Outcast!, 1999; released Lucy Ford: The Atmosphere EPs, 2001; God Loves Ugly, 2002; Seven's Travels, 2003; You Can't Imagine How Much Fun We're Having, 2005; When Life Gives You Lemon, You Paint That S--t Gold, 2008.

Addresses: Record company—Rhymesayers Entertainment, 2411 Hennepin Ave. S., Minneapolis, MN 55404, Web site: http://www.rhymesayers.com. Publicist—Biz 3 Publicist, 1321 N. Milwaukee Ave., #452, Chicago, IL 60622.

Selected discography

Overcast!, Rhymesayers, 1998.

Lucy Ford: The Atmosphere EP's, Rhymesayers, 2001.

God Loves Ugly, Rhymesayers, 2002.

Seven's Travels, Rhymesayers, 2003.

You Can't Imagine How Much Fun We're Having, Rhymesayers, 2005.

When Life Gives You Lemons, You Paint That S--t Gold, Rhymesayers, 2008.

Sources

Periodicals

Village Voice, June 26, 2002; November 3, 2004.

Online

"Atmosphere: Seven's Travels," Rolling Stone,http://www.rollingstone.com/artists/atmosphere2/albums/album/300124/review/5943244/sevens_travels (February 10, 2008).

"Atmosphere: You Can't Imagine How Much Fun We're Having," The A.V. Club,http://www.avclub.com/content/node/41761 (February 10, 2008).

"The Fifth Element of Hip-Hop," Synthesis,http://www.synthesis.net/music/interview/item-2368/2002-09-17-the_fifth_element_of_hip-hop (February 10, 2008).

Rhymesayers Entertainment Official Web site, http://www.rhymesayers.com (February 10, 2008).

"Slug," The A.V. Club,http://www.avclub.com/content/node/57340 (February 10, 2008).

—Shannon McCarthy

27. Atmosphere

See also 85. CLIMATE ; 87. CLOUDS ; 142. ENVIRONMENT ; 417. WEATHER ; 420. WIND .

advection

the horizontal movement of elements of the atmosphere. Cf. convection . —advective, adj.

aerodynamics

the branch of dynamics that studies the motions of air and other gases, especially with regard to bodies in motion in these substances. See also 31. AVIATION . —aerodynamic, aerodynamical, adj.

aerographics, aerography

the branch of meteorology that studies and describes atmospheric conditions. —aerographer, n. —aerographic, aerographical, adj.

aerology

1. Obsolete, the branch of meteorology that observed the atmosphere by using balloons, airplanes, etc.

2. meteorology. —aerologist, n. —aerologic, aerological, adj.

aeromancy

1. divination from the state of the air or atmospheric conditions, sometimes limited to weather.

2. Humorous. weather forecasting. See also 124. DIVINATION.

aerometry

the science of measuring properties of air; pneumatics. —aerometric, adj.

aeropause

the region in the upper part of the earth’s atmosphere where the air is too thin for aircraft to operate properly.

aerophobia

an abnormal dread of fresh air. —aerophobe, n.

aeroscepsy, aeroscepsis

perception by means of the air, said to be a function of the antennae of insects.

aerosphere

Aeronautics. the area outside the atmosphere of the earth where manned flight is possible.

atmolysis

the separation of gases which are equally diffusible. —atmolyzer, n.

atmospherics

1. the sound, usually a crackling noise, heard over a radio receiver and caused by electromagnetic disturbances in the atmosphere; static.

2. the natural phenomena that create this disturbance.

barograph

a barometer which automatically records, on a rotating cylinder, any variation in atmospheric pressure; a self-recording aneroid.

barometrography

the branch of science that deals with the barometer.

barometry

the art or science of barometric observation.

bioclimatology

a branch of biology that studies the relationship between living creatures and atmospheric conditions. Also called biometeorology . —bioclimatologist, bioclimatician , n. —bioclimatological, adj.

chaomancy

a form of divination involving aerial visions.

convection

the vertical movement of elements of the atmosphere. Cf. advection .

eudiometer

an instrument for measuring the amount of oxygen in the air and for analyzing gases.

exosphere

the highest portion of the earth’s atmosphere, from which air molecules can escape into space. Cf. ionosphere .

ionosphere

the outermost part of the earth’s permanent atmosphere, beyond the stratosphere, composed of heavily ionized molecules. It extends from about 50 to 250 miles above the surface of the earth. Cf. exosphere .

konimeter

an instrument for measuring impurities in the air. —konimetric, adj.

konimetry

the measurement of impurities in the air by means of a konimeter. —konimetric, adj.

koniology, coniology

the study of atmospheric dust and other impurities in the air, as germs, pollen, etc., especially regarding their effect on plant and animal life.

miasmology

the study of fogs and smogs, especially those affecting air pollution levels.

microbarograph

a barograph for recording small fluctuations of atmospheric pressure.

ozonometry

the determination of the proportion of ozone in the atmosphere. —ozonometer, n. —ozonometric, adj.

pneumatics

a specialty in physics that studies the mechanical properties of air and other gases. Also called pneumodynamics .

stratosphere

the upper part of the earth’s atmosphere, characterized by an almost constant temperature throughout its altitude, which begins at about seven miles and continues to the ionosphere, at about 50 miles.

sympiesometer, sympiezometer

an instrument for measuring the weight of the atmosphere by the compression of a column of gas. See also 226. INSTRUMENTS .

tropopause

the zone between the troposphere and the stratosphere where the temperature remains relatively constant above a given point on earth.

troposphere

the region of the earth’s atmosphere between the surface of the earth and the stratosphere.

vacuometer

an instrument used for comparing barometers at varying pressures against a standard barometer.

ATMOSPHERE

The earth's atmosphere is simple in some respects, and complex in others. It is relatively uniform in composition with respect to its major mass components (oxygen and nitrogen), yet extremely variable in some minor components, such as water vapor and ozone (O₃), which play major roles in its heat and radiation fluctuations. The atmosphere has a complex structure based on temperature gradients. This structure governs its mixing characteristics and the buildup of contaminants, yet is usually invisible, except when light-scattering particles suspended in the air make it visible. The structure of the atmosphere is of major importance to the dilution and dispersion of contaminants. It is governed by the lapse rate, which is the rate of change of air temperature with height above the ground.

The lowest of the atmospheric layers is the troposphere, which contains about 75 percent of the mass of the atmosphere, and almost all of its moisture. It extends to a height that varies from about 9 kilometers at the poles to about 15 kilometers at the equator, and it has an average lapse rate of about −6.5°C/km. The boundary between the troposphere and the next layer, the stratosphere, is known as the tropopause. The stratosphere contains essentially all of the remainder of the mass of the atmosphere; it is nearly isothermal (the temperature does not change with altitude) in the lower regions and shows a temperature increase with height in the upper regions. There is very little air exchange between the well-mixed and turbulent troposphere and the nearly stagnant stratosphere.

The major constituents of dry air at ground level are nitrogen (N₂) at 78.1 percent by volume, oxygen (O₂) at 21.0 percent, and argon (Ar) at 0.9 percent. Carbon dioxide (CO₂) is present at about 330 ppm by volume and methane (CH₄) at about1.5 ppm by volume. About 3 percent of the total mass of the lower atmosphere is water vapor (H₂O), but the concentration is extremely variable in both space and time. In general, the warmer portions of the atmosphere contain more water vapor. The water vapor content becomes lower with increasing altitude and with increasing latitude. Water vapor plays a critical role in governing the earth's heat exchange and the motion of the atmosphere, due to its high heat capacity, absorption of infrared radiation, and heat of vaporization. Further effects attributable to atmospheric water result when air motion creates clouds (aerosols of water droplets), in which the energy received as sunshine in one place is liberated as the latent heat of vaporization in another.

Of the incoming radiant energy, about 30 to 50 percent is scattered back toward space, reflected primarily by clouds and, to some extent, by solid particles or by the earth's surface. About 20 percent of the incident radiant energy is absorbed as it passes through the atmosphere. Stratospheric O₃ absorbs about 1 to 3 percent, primarily in the short-wave ultraviolet (UV) portion of the spectrum; this effectively limits further penetration to those wavelengths greater than 0.3 microns. In the troposphere, 17 to 19 percent of the incoming radiation is absorbed, due primarily to water vapor and secondarily to CO₂.

The average radiation into space essentially equals that absorbed from the sun, and a substantial amount of energy must flow from the tropics toward the poles within the oceans and the troposphere. This flow of energy is accomplished primarily by systems of warm air and ocean currents that flow toward the poles and cool currents that flow toward the tropics.

The dispersion of contaminants within the atmosphere is generally referred to as diffusion. For practical purposes, the dispersion of contaminants by molecular diffusion is negligible because the extent of movements are generally infinitesimal compared to the movements of the air volumes containing them by the turbulent motions of the air (turbulent diffusion).

Atmospheric turbulence is a complicated phenomenon that has defied mathematical description. When considering contaminant dispersion, contaminant sources can be divided into three different categories: (1) point sources, such as tall industrial smokestacks; (2) line sources, such as highways; and (3) area sources, such as whole urban regions. The simplest is an elevated point source. The light-scattering properties of the aerosol in the plume from such a stack, consisting of fly ash and condensed water, enable us to observe plume dispersion with the unaided eye.

The vertical mixing of air is dependent upon the temperature profile of the atmosphere (the lapse rate). The immediate ground level concentrations of air contaminants may be reduced by vertical mixing, since dispersal into higher regions dilutes the contaminants. Poor vertical mixing may allow concentrations released at low altitudes to remain there in relatively concentrated form. An extreme case of atmospheric stability occurs when the atmospheric lapse rate is negative (when the temperature increases with altitude). This condition is known as a temperature inversion. There is virtually no vertical air movement within inversion layers and contaminants accumulate within them.

Morton Lippmann

(see also: Airborne Particles; Ambient Air Quality [Air Pollution]; Climate Change and Human Health )