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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