Chapter 2: The National Wind Resource

This chapter describes the geographical distribution of the wind energy resource throughout the United States and its territories, the certainty credited to the wind resource estimates, and the areal distribution (percentage land area) of the wind resource. Two types of national wind resource maps are provided: analyzed (Maps 2-1 through 2-5) and gridded (Maps 2-6 through 2-25). Five fold out analyzed maps of the annual and seasonal average wind resource precede 20 gridded maps at the end of this chapter. Gridded maps are of the annual and seasonal average wind resource, the certainty rating of the resource estimates, and the areal distribution of the resource. They are shown for the contiguous United States, Alaska, Hawaii, Puerto Rico, and the Virgin Islands. Grid cells are 1/4° latitude by 1/3° longitude in the contiguous United States, 1/2° latitude by 1° longitude in Alaska, and 1/8° latitude by 1/8° longitude in Hawaii, Puerto Rico, and the Virgin Islands.
Because of the large areal extent of the Pacific Islands and the sparseness of the data for these islands, no wind resource information was digitized for inclusion in the gridded maps. Also, these islands are not shown on the analyzed maps (2-1 through 2-5), although a brief description of the estimated wind resource for these islands is included in the map description on analyzed Map 2-1. For information on the Pacific Islands, refer to Chapter 3 for maps and descriptions of these areas. Also, refer to the wind energy atlas (Volume 11) covering the Pacific Islands.
Chapter 1 provides information on how to interpret these maps. In the following discussions about the wind power maps, many references are made to specific geographic locations. Refer to the regional and state maps in Chapter 3 (Maps 3-1 through 3-72) to identify unfamiliar locations.
Annual Average Wind Resource
Areas that are potentially suitable for wind energy applications (wind power class 3 and above) are dispersed throughout much of the United States (Maps 2-6 and 2-16). Major areas of the United States that have a potentially suitable wind energy resource include: much of the Great Plains from northwestern Texas and eastern New Mexico northward to Montana, North Dakota, and western Minnesota; the Atlantic coast from North Carolina to Maine; the Pacific coast from Point Conception, California to Washington; the Texas Gulf coast; the Great Lakes; portions of Alaska, Hawaii, Puerto Rico, the Virgin Islands, and the Pacific Islands; exposed ridge crests and mountain summits throughout the Appalachians and the western United States; and specific wind corridors throughout the mountainous western states.
In the Great Plains, class 5 wind resource is found over elevated areas of North Dakota, such as the Pembina and Missouri escarpments and Turtle Mountains, and the hilltops and uplands of the Missouri Plateau in southwestern North Dakota and high plains in northwestern Montana near Cut Bank. Class 4 wind resource exists over hilltops and uplands of eastern Montana and high plains in northwestern Montana, much of North and South Dakota, the Sand Hills of Nebraska, western Minnesota, northwestern Iowa, the Texas Panhandle, northwestern Oklahoma, southcentral Kansas and the Flint Hills of eastern Kansas, uplands of eastern Colorado, and parts of northeastern New Mexico.
Exposed coastal areas in the Northeast from Maine to New Jersey and in the Northwest southward to northern California indicate class 4 or higher wind resource. Class 4 or higher wind resource also occurs over much of the Great Lakes and coastal areas where prevailing winds (from the strong southwest-to- northwest sector) have a long, open-water fetch. Class 3 wind resource can be found along exposed coastal areas from Delaware to North Carolina, much of the California coast north of Point Conception, and the Texas coastal areas from the Mexican border northward to Galveston. Along many coastal areas, the abrupt increase of surface roughness inland from the coastline because of vegetation and topography can rapidly attenuate the wind resource inland. Notable exceptions occur along the Texas coast and Cape Cod in Massachusetts where the coastal wind resource extends inland a considerable distance.
Many of the higher exposed ridge crests and mountain summits in the eastern and western United States experience high wind resource, because mean upper-air wind speeds are strong over most of the contiguous United States during much of the year. However extreme winds, icing, and inaccessibility caused by poor weather and snow depths during the winter severely restrict the suitability of many of these areas for wind energy development.
In basins, valleys, and lowland plains throughout the mountainous regions, mean annual wind power is generally low. During colder months, cold air often fills the basins and valleys, creating a vertical temperature profile that frequently remains stable throughout the day because of low insolation. Under these stable surface conditions, vertical mixing of the atmosphere is limited, and light surface winds usually persist in the lowland areas, even though winds may be strong on nearby higher terrain. In warmer months, although insolation and vertical mixing increase, mean wind speeds aloft are much lower than in colder months.
However, high wind resource at relatively low elevations in mountainous regions can occur where the air flow is channeled through constrictions or corridors that enhance the wind speeds. These wind corridors vary in width from just a few kilometers to over 50 km (31 mi). On the national maps, most of these wind corridors appear relatively small in geographical extent and many are hardly noticeable among the vast expanse of mountain ranges in the western United States. However, because many of these wind corridors serve as primary transportation corridors, they are easily accessible, in contrast to the higher mountain summits and ridge crests. Moreover, weather conditions are not nearly as severe in these corridors as they are on the higher mountain ranges. Thus, considerable activity in wind energy development is taking place in many of these wind corridors in the western United States. However, smaller scale terrain features within these corridors, combined with the larger scale channeling effects, can cause extreme local variability throughout many of these corridors and complicate the siting process.
Some notable corridors where class 4 or higher wind resource can be found are located in California, Oregon, Washington, Montana, and Wyoming. Isolated corridors with high resource may occur in some of the other states where mountainous terrain exists. In California, several corridors through the Coast Range occur from east of San Francisco southward to San Diego. Some of the notable corridors in California shown on the national annual average wind power map are in the areas of San Gorgonio, Tehachapi, Altamont and Pacheco Passes and the Carquinez Straits. In addition to these passes, high wind resource occurs over some of the lower ridges of the Coast Range in southern California. In Oregon and Washington, the two most notable corridors are the Columbia River corridor, which extends about 200 km (124 mi) eastward from Portland, Oregon, and the corridor in the vicinity of Ellensburg, Washington. In Montana, high wind corridors occur in the areas of Livingston, Whitehall and Harlowton-Judith Gap. In Wyoming, a broad gap over 100 km wide (62 mi) through the Rocky Mountains creates the vast wind corridor of high wind resource in southern Wyoming.
In Alaska, high wind resource (up to class 7) occurs over the Aleutian Islands, much of the coastal areas of northern and western Alaska, offshore islands in the Bering Sea and Gulf of Alaska, and over mountainous areas in northern, southern, and southeastern Alaska. Basins and valleys in interior Alaska generally have class 1 or 2 wind resource. A few corridors in interior Alaska are estimated to have high wind resource.
In Hawaii, interactions between prevailing trade winds and island topography determine the distribution of wind power. On all major islands, trades accelerate over coastal regions at the island corners. The best examples are regions of class 6 or higher wind power on Oahu, Kauai, Molokai, and Hawaii.
In Puerto Rico, class 3 or 4 wind resource is possible at sites along the northern and eastern coasts, which are well exposed to the prevailing trade winds, and at higher peaks and ridges in the interior.
The Virgin Islands are shown on the gridded map but not on the analyzed map. Wind resource of at least class 3 is possible at well-exposed sites on the central ridges, the northern, eastern, and southern coasts of St. Thomas, St. John, and St. Croix, as well as the windward sides of the smaller islands.
For the Pacific Islands, which are not shown on either the gridded or analyzed annual maps, please refer to Chapter 3 for maps and descriptions of these islands.

Seasonal Variations of the Wind Resource
Because there is considerable seasonal variation in the wind energy resource, with maxima in winter and spring and minima in summer and autumn throughout most of the contiguous United States, assessments of the wind energy resource have also been produced for each season. The geographical distribution of the wind resource throughout the nation is portrayed for each of the seasons in Maps 2-12 through 2-15 and 2-22 through 2-25. The Pacific Islands are not shown on the gridded or analyzed maps. However, a discussion of the seasonal variations of the wind resource for these islands based on wind power values estimated from ship wind data is included in this chapter. For further information on these islands, refer to Chapter 3 of this atlas and Volume 11 of the wind energy atlas covering the Pacific Islands. The season of maximum wind energy is winter in most of Alaska and many of the Pacific Islands, and summer in Hawaii, Puerto Rico, and many of the Virgin Islands. A substantial portion of the United States has class 3 or higher wind resource in spring and winter, whereas a considerably smaller portion has class 3 and above wind resource in summer. The distribution of wind resource throughout the United States in winter, spring, summer, and autumn is described more completely in the following four sections.

Winter Wind Resource (December, January, February)
In winter, mean upper-air wind speeds are stronger than in any other season over most of the contiguous United States. Class 3 and above wind resource can be found at exposed sites throughout most of the contiguous United States except for the southeastern United States (excluding ridge crests), much of southern Texas, the basins and valleys of the western United States, and heavily forested areas and sheltered valleys and basins of the northeastern United States. Over the northern Great Plains, class 5 wind resource is found in winter over portions of North and South Dakota. Class 5 and 6 resource occurs over portions of the high plains in northwestern Montana from Great Falls to the Canadian border. Class 4 wind resource covers a substantial part of the northern Great Plains, including much of the Dakotas, hilltops and uplands of eastern Montana, and the Sand Hills of Nebraska. The class 4 wind resource extends eastward into western and southern Minnesota and much of Iowa, hilltops and uplands in southwestern Wisconsin, and a portion of central Illinois.
Over the southern Great Plains, class 4 is prevalent over a portion of the Texas Panhandle, northwestern Oklahoma, and southcentral Kansas. Class 4 also occurs over the Flint Hills of eastern Kansas, portions of eastern Colorado, and extreme northwestern Kansas, and hilltops in northeastern New Mexico. A band of class 4 is estimated to exist along elevated areas of the Ozark Plateau in southern Missouri and over ridge crests and mountain summits of the Boston and Ouachita mountains in western Arkansas and eastern Oklahoma.
Exposed coastal areas in the Northeast and Northwest have class 5 or above wind resource in winter. Large portions of the Great Lakes shorelines and islands are estimated to have class 5 or 6 wind resource in winter. Class 3 or 4 wind resource can be found in winter along the coastal areas of much of central and northern California, North Carolina, Virginia, Texas, parts of Louisiana, and the Florida Keys.
In the East, from Tennessee and North Carolina northward to Maine, many exposed uplands, hilltops, and lower mountain summits are estimated to have class 4 wind resource in winter.
Many of the higher exposed ridge crests and mountain summits in the eastern and western United States experience as much as class 7 wind resource for a winter average. However, extreme winds, icing, and inaccessibility caused by poor weather and snow depths during winter severely restrict the suitability of many of these areas for wind energy development.
Although mean upper-air wind speeds are strongest in the winter, mean wind speeds are generally low in basins, valleys, and lowland plains throughout the mountainous regions. Cold air often fills the basins and valleys, creating a vertical temperature profile that frequently remains stable throughout the day because of low insolation. Under these stable surface conditions, vertical mixing of the atmosphere is limited, and light surface winds usually persist in the lowland areas, even though winds may be strong on nearby higher terrain. Thus, basins, valleys, and lowlands throughout the mountainous regions generally have only class 1 or 2 wind resource in the winter.
However, high wind resource in the winter can occur in areas where cold air drainage from higher elevations to lower elevations is channeled through constrictions or corridors that enhance the wind speeds. These wind corridors vary in width from just a few kilometers to over 50 km (31 mi). Highest wind speeds are usually near the corridor outlets. Wind corridors that have class 3 and above wind resource in the winter are located near Portland (the western part of Columbia River gorge) and La Grande, Oregon; Strevell, Idaho, near the Idaho-Utah border, about 120 km (75 mi) southeast of Twin Falls; Whitehall, Livingston, and Judith Gap, Montana; Cody, Wyoming; Santa Fe, New Mexico; and Milford, Utah. Several corridors are found in southern and central Wyoming, where prevalent high wind speeds are channeled and enhanced. An example of this is the area around Medicine Bow, Wyoming, where prevailing westerly winds are channeled between the Medicine Bow Mountains to the south and the Shirley Mountains to the north. This area has class 7 wind resource in the winter.
Throughout most of Alaska, winter is the season of maximum wind power. Areas with winter maxima include all of the southeast and southwest subregions, all mountain areas, and the west coast of southcentral Alaska. Very high wind resource (class 6 and 7) in winter occurs over the Aleutian Islands, much of the coastal areas of northern and western Alaska, offshore islands in the Bering Sea and Gulf of Alaska, and over some mountainous areas in southern and southeastern Alaska. Basins and valleys in interior Alaska generally have only class 1 or 2 wind resource. A few corridors in interior Alaska where the winds are channeled and enhanced have high wind resource in winter.
In Hawaii during winter, the trade winds are less frequent, though migratory anticyclones can produce strong trades for prolonged periods. Low pressure systems and intense cold fronts occasionally produce strong southwesterly and westerly winds. However, these systems do not occur often enough to alter the basic power density distributions. Wind power is greatest on coastal corners exposed to prevailing trade winds. Each island has some area of class 6 wind power. The Kohala and South Point areas on the island of Hawaii experience wind power of class 7 as does Ilio Point on northwestern Molokai.
For the Pacific Islands, winter is the season of maximum wind power over much of the region. Winter in American Samoa is June through August. Except for Guam (the largest Pacific Island), seasonal wind power values are presented for the surrounding ocean areas only. Cold air outbreaks from the Asian winter monsoon produce strong trade winds over the western North Pacific. Very high wind resource (class 6 and 7) is estimated for the Marshalls, the Northern Marianas and the ocean area around Guam. Class 4 wind resource is estimated for the southern mountains of Guam, while class 3 power is estimated for the rest of the island. Class 3 and 4 wind resource is estimated for the Carolines, which are located away from the major winter trade wind belts. Class 4 power is estimated for American Samoa, which is exposed to winter trade winds. Wake, Johnston, and Midway are estimated to have class 6 and 7 wind power.
Over Puerto Rico in winter, class 4 wind resource is estimated for the higher peaks and ridges in the interior. Class 3 wind resource is predominant at sites along the northern and eastern coasts, which are well exposed to the prevailing trade winds.
Over the Virgin Islands in winter, class 3 wind resource is estimated for exposed sites on the northern and eastern coasts. Class 4 is estimated for some of the higher ridge crests on St. Thomas and St. John.

Spring Wind Resource (March, April, May)
In spring, the mean upper-air flow is weaker than in winter but remains quite strong over most of the contiguous United States, although its strength decreases as spring progresses from March to May. Thus, in spring the wind resource is generally less than in winter on mountain summits and ridge crests (except in the extreme southern part of the Southwest) and exposed coastal areas of the Northwest, Northeast, and Great Lakes.
Because incoming insolation is greater in spring than in winter, temperature profiles are less stable, and more vertical mixing in the surface layer results than in winter. Therefore, near-surface mean wind speeds over the valleys, basins, and plains throughout most of the United States west of the Mississippi River are generally greater in spring than in winter. In the eastern third of the United States, mean wind speeds over the plains, basins, and valleys in spring are about the same magnitude as in winter or only slightly less, even though mean upper-air wind speeds are considerably greater in winter than in spring.
In spring, the coastal regions exhibit the greatest thermal contrasts between land and sea. The combined effects of weakened, but still significant, upper-air flow and regional, thermally induced flow in the coastal areas produce wind powers in the spring that exceed those in winter along much of the California coast and south Texas coast and are comparable to those in winter along much of the southern Atlantic coast, the Gulf coast, and the coastal areas of the western Great Lakes.
In spring, class 3 and above wind resource occurs at exposed areas throughout much of the United States, except the southeastern United States where class 3 and above is restricted to exposed mountain summits and ridge crests in the Appalachians and coastal areas from North Carolina northward.
Over much of the central United States from eastern Montana to Minnesota and south to Texas, wind power reaches a maximum in the spring. Areas of highest wind resource over this region, class 6, occur in the northern Great Plains over elevated escarpments and uplands throughout North Dakota, near Rapid City in South Dakota, and uplands near Circle, 110 km (70 mi) north of Miles City in eastern Montana. Class 5 occurs over the high plains of the Texas Panhandle, northwestern Oklahoma, and southcentral Kansas.
Areas of southern and central Wyoming and northwestern Montana that had class 6 and 7 in winter decrease by 1 to 2 power classes in spring.
Exposed coastal areas along the Pacific coast (north of Point Conception, California) have class 4 power in spring, and the wind power is accelerated to class 5 around more prominent capes such as Cape Blanco, Oregon, and Cape Mendocino, California. Exposed coastal areas of the Northeast (from North Carolina north to Maine) have class 4 and 5 power, increasing to class 6 over Cape Cod and Nantucket Island, Massachusetts. Class 4 and 5 resource occurs over much of the Great Lakes and their exposed coastal areas.
Along the south Texas coast, wind power in spring increases inland from class 3 over the outer coastal increased convection from greater solar heating. These factors reduce the wind power at exposed mountain locations from class 4 in winter to class 3 in spring.
Over the Virgin Islands, the trade winds weaken slightly in spring; thus, only class 2 wind power is typical of exposed coastal locations on the windward sides of the three main islands and the smaller islands. Class 3 wind power is estimated for some of the exposed ridge crests on the islands.

Summer Wind Resource (June, July, August)
In summer, wind speeds aloft diminish, and wind power is at its lowest over most of the United States. Although only class 1 or 2 wind power occurs over much of the contiguous United States, areas of class 3 or higher wind resource occur over much of the northern and southern Great Plains, the Great Lakes, the south Texas coast, the Pacific coast from southcentral California northward to Oregon, southern Wyoming, the wind corridors in specific areas of California, Oregon, Washington, Montana, and Utah, and exposed mountain summits and ridge crests throughout the West. In the Northeast, class 3 wind power in summer can be found over Cape Cod and Nantucket Island, Massachusetts, and exposed ridge crests in Vermont, New Hampshire, and Maine.
Summer is the season of maximum wind energy in Hawaii, Puerto Rico, the Virgin Islands, and parts of California, Oregon, and Washington. In these regions, specific areas have high wind energy resource in the summer.
Along the West Coast, class 3 or 4 wind resource occurs at exposed coastal areas from Point Conception, California, north through Oregon. Persistent, strong north-to-northwest winds, which occur during summer along much of the West Coast, are associated with the summer anticyclone (high-pressure system) over the eastern Pacific Ocean. The southern California coastline south of Point Conception has low wind power potential, because it is sheltered from the strong northwest winds by the Transverse Range. Major coastal capes that protrude into northerly flow experience the highest power, such as Cape Blanco and Cape Mendocino. Concave coastal areas, which are typically located between the protruding capes, typically have low-to-marginal wind resource (class 1 to 2) because they are sheltered from the strong northerly winds. The abrupt increase in surface roughness inland from the coastline, because of vegetation and topography, further slows the wind.
High wind resource in the Pacific coast states occurs inland where strong surface-pressure gradients created by the cold water and warm interior force marine air through the major gaps in the mountains into the interior. Strong, persistent winds occur during most of the summer in these wind corridors. Areas of class 6 or 7 wind resource exist in summer where the topography funnels or enhances the flow in these wind corridors. Several wind corridors of this nature occur in California, such as Carquinez Straits, and Altamont, Pacheco, Tehachapi, and San Gorgonio Passes. Two major wind corridors in the Northwest where areas of high wind resource occur in summer are the Columbia River corridor along the Oregon-Washington border and the Ellensburg corridor in Washington.
In Alaska, although summer is the season of minimum wind power, class 3 and higher wind power can be found along the Arctic coast, the western coast and islands offshore, over the Alaska Peninsula and Aleutian Islands, Kodiak Island, and at a few interior locations. Some of the Aleutians and well-exposed capes on the western coast of Alaska even have class 6 or 7 wind resource in summer, the season of lowest wind resource.
In Hawaii, summer is the season of maximum trade wind frequency and, in most regions, maximum wind power. Trade wind steadiness (defined as the ratio of resultant mean speed to mean wind speed) is typically 90%. In each county, some regions experience class 7 wind power and significant sections class 6. These summer trade winds are probably the steadiest wind power source in the United States.
For the Pacific Islands, summer is the season of minimum wind power over much of the region, with the exception of Johnston Island where strong summer trade winds indicate class 6 wind resource. Class 4 and 5 wind power is estimated for the central and northern Marshalls, while class 3 wind power is estimated for the Northern Marianas and the ocean area around Guam. Wind resource of only class 2 is estimated for the mountains of Guam with class 1 power estimated for the rest of the island. The near-equatorial trough dominates summer weather over the Carolines and the southern Marshalls where the wind resource is estimated to be only class 1. The monsoonal trough of northern Australia extends eastward over Samoa where class 2 wind power is estimated. Wind power at Wake and Midway is estimated to be class 4 and 3, respectively.
Over Puerto Rico, the summer trade winds are well developed throughout the lower atmosphere, making this the season of maximum wind power for most of Puerto Rico. Class 4 wind power can be found at exposed coastal sites on the northern and eastern coasts of Puerto Rico, on the windward sides of the outlying islands, at the highest mountain tops in Puerto Rico and on the exposed hilltops of Culebra and Vieques.
Over the Virgin Islands, summer is also the season of maximum wind power, as trade winds are well-developed throughout the lower atmosphere. Class 4 wind power is estimated for the ridgelines of St. Thomas and St. John, the highest hills on St. Croix, at well-exposed coastal locations, and on the eastern sides of the smaller islands.

Autumn Wind Resource (September, October, November)
In autumn, upper-air wind speeds increase as autumn progresses toward winter. Consequently, the mean wind power is considerably greater in November than in September over much of the country. Throughout most of the contiguous United States, the mean autumn wind resource is less than that of spring and winter but greater than that of summer.
In the contiguous United States, class 3 or greater wind resource in autumn occurs along the coastal areas of the Northeast (from Cape Hatteras northward), Northwest, Great Lakes, and a portion of the Texas coast; exposed mountain summits and ridge crests throughout the Appalachians and western! mountains; most of the Great Plains from northern Texas to North Dakota and Montana; and high plains and wind corridor areas in Montana and Wyoming. Some of the wind corridors in California continue to have high wind resource into the autumn.
In Alaska, autumn is the season of maximum wind power along much of the Arctic coast of northern Alaska, which experiences class 6 and 7 average wind power in the autumn. During this season there are more frequent migratory storms, and there is often open water early in the season. Some of the most severe storm surges on the Beaufort coast have occurred in September and October. By the middle of November, the sea ice generally has completely covered both the Chukchi and Beaufort seas, reducing the temperature contrast (and hence storm intensities) along the coasts. In other areas of Alaska, high wind resource in autumn occurs throughout the Aleutian Islands and most coastal areas of western and southern Alaska, although the wind resource in autumn is generally less than that in winter in these areas.
In Hawaii, autumn is a transition period marked by a gradual weakening of the North Pacific anticyclone and the first southward advances of cold fronts. Winds are weaker in autumn than in summer throughout the state. Nevertheless, the Kahuku region of Oahu and the Kohala mountains of Hawaii continue to experience class 7 wind power. The most dramatic wind power decrease is in northeastern Kanai, where Kilauea Point drops from a summer rating of class 7 to class 3 in autumn. Even in this relatively weak wind season, regions of class 6 wind power densities exist in each county.
Over the Pacific Islands, the weakened winds of summer persist into autumn except for the Northern Marianas, and Wake, Johnston, and Midway Islands where ship winds indicate that up to class 6 wind resource may be present. On Guam, class 3 wind power is estimated for the southern mountains with class 1 power estimated for the rest of the island. Class 3 wind power is estimated for the northern Marshalls, while only class 1 and 2 wind resource is estimated for the rest of the Pacific Islands.
Over Puerto Rico and the Virgin Islands, there is a marked decrease in the strength of the trade winds in the autumn. In addition, sea-land temperature differences are less, thus reducing the sea breeze. These factors combine to make autumn the season of minimum power. Only Cape San Juan, because of its excellent exposure, experiences class 3 wind power.

Certainty of the Resource Estimates
The degree of certainty with which the wind power class can be specified depends on three factors: the abundance and quality of data; the complexity of terrain; and the geographical variability of the resource (Appendix A has a more complete description of certainty rating). A certainty rating of the energy resource estimate from 1 (low) to 4 (high) has been made for each cell of a 1/4° latitude by 1/3° longitude grid in the contiguous United States, 1/2° latitude by 1° longitude in Alaska, and 1/8° latitude by 1/8° longitude in Hawaii, Puerto Rico, and the Virgin Islands.
Maps 2-7 and 2-17 show the certainty rating of the wind resource estimates for the United States. The largest area of certainty rating 4 in the contiguous United States occurs over the southeastern plains, from eastern North Carolina southward to Florida and westward to eastern Texas. A combination of factors (such as abundant surface wind data from exposed locations, tower wind data at levels of 50 m to 100 m (164 to 328 ft) above ground, small variability in the wind energy resource, and mostly flat to rolling terrain) indicate that this region of the country has low wind energy potential, with a high degree of confidence, for current wind turbine applications. Throughout this region, existing data indicate only class 1 wind power in the interior areas and only class 2 at exposed coastal areas from Louisiana to Florida and Georgia.
Another area of generally high certainty ratings occurs in the upper Midwest from Illinois eastward to western Ohio and southern Michigan. High certainty ratings have also been assigned to some of the major metropolitan areas in the Northeast. The wind resource estimates for much of the upper Midwest and Northeast are primarily based on abundant surface data from airfields and data from meteorological towers, ranging from 30 m to 200 m (98 to 656 ft) above ground, collected by utilities.
Areas of high certainty or high-intermediate certainty have been assigned to specific areas along the Great Lakes shorelines and the Northeast coast where the wind resource estimates are based on data collected near 50 m (164 ft) above ground and/or well exposed sites with data near 10 m (33 ft). For example, high certainties have been assigned to the grid cells in the vicinity of the DOE candidate sites at Montauk Point, New York, and Block Island, Rhode Island, because the wind resource values for these areas are based on approximately five years of wind measurements, 45.7 m (150 ft) above ground at well-exposed sites.
Over the Great Plains (from northern Texas and eastern New Mexico northward to the Dakotas), areas of highest certainty indicate specific areas where the wind resource estimates are based on wind data collected at or near the 50-m (164 ft) level at exposed sites. Usually, these are sites with two years or more wind data, where meteorological towers were instrumented specifically for wind energy assessment purposes. DOE instrumented many of these sites while others were established by the Alternative Energy Institute, Kansas State University, or other organizations. Areas over the Great Plains with high-intermediate certainty (rating 3) generally indicate areas where wind resource data exist at or near 10 m (e.g., 4 to 20 m above ground or 13 to 66 ft) at exposed sites and/or where limited wind data exist near 50 m (164 ft). Because of some uncertainty in the nature of the wind shear profile at specific sites, the wind resource at 50 m (164 ft) cannot be reliably estimated, with high confidence, from data collected near 10 m (33 ft). For example, in some areas of the Great Plains, the nocturnal wind shear is very strong such that these areas exhibit a strong nighttime maximum and daytime minimum in the wind resource at 50 m (164 ft). In other areas of the Great Plains, this is not the case, as the height of transition is considerably higher up. Existing data from meteorological towers in different areas of the Great Plains show considerable variation in the wind shear profiles.
In the West, high certainty areas are more sparse as a result of the overall greater complexity of the terrain and lack of data in many areas. Even in many areas of the West where considerable data exist, such as Los Angeles and San Francisco, California, and Denver, Colorado, the large spatial variability in the wind resource eliminates a high certainty rating. Two large areas in the West with a high certainty rating are the San Joaquin and Sacramento valleys in California and the Snake River valley in Idaho.
Most of the mountainous areas of the United States have certainty ratings of 1 or 2, as these areas generally had little representative surface data and estimates for summits and ridge crests were primarily derived from free-air measurements (e.g., weather balloons).
Over Alaska, certainty ratings are mostly low (1 and 2), primarily because of the complexity of the terrain over most of the state and sparsity of data in many areas. However, some areas with high wind resource and high certainty exist where representative surface data were available.
Over Hawaii, the distribution of certainty rating varies considerably from island to island. Certainty ratings are mostly 2 and 3 over Oahu and the island of Hawaii, with certainty 4 in the vicinity of Honolulu, Hilo, and Kona. Much of Maui and Molokai has complex terrain and/ or no historic data and, for these reasons, has mostly certainty ratings of 1 and 2. In Kauai, ratings vary from 1 over the central mountains and northwest coast to 4 at Lihue.
Over Puerto Rico, the wind power estimates for most of the coastline perimeter have a certainty rating of 3 because of the quantity of wind data and the predictable nature of the trade winds near the coastlines. Wind power in the entire mountainous interior of Puerto Rico has been assigned a certainty rating of 1, as there were no wind data from exposed sites in the mountainous areas.
Most of the Virgin Islands have been assigned a relatively low certainty of 2 as a result of the lack of data and the complex terrain of these islands.
For the Pacific Islands, refer to Volume 11 of the regional wind energy atlases, Hawaii and the Pacific Islands (Shroeder et al. 1981), for maps and discussion of the certainty ratings. Information for the Pacific Islands was not digitized, because the islands are dispersed throughout vast areas of the Pacific Ocean.
Maps 2-8, 2-9, 2-18, and 2-19 show the certainty of the wind resource estimates in the United States for those areas estimated to have an annual average wind resource of class 3 or greater and class 4 or greater, respectively. Only a small fraction of the areas estimated to have class 4 or greater wind resource can be assured, with high certainty, of having that resource. Except for the Great Plains, most of the high wind resource estimates are in mountainous, hilly, or coastal areas where there is considerable spatial variability in the wind resource. Especially in mountainous terrain, there was usually little surface data to verify the resource estimates based largely on upper-air wind data.

Areal Distribution of the Wind Resource
Because the wind power class values shown on the wind resource maps apply only to areas well exposed to the wind, the map area does not indicate the true land area experiencing this power. The fraction of the land area represented by the wind power class shown on the maps depends on the physical characteristics of the land-surface form. On a flat open plain, for example, close to 100% of the area will have a similar wind power class, while in hilly and mountainous areas the wind power class will only apply to a small proportion of the area that is well exposed.
The areal distribution of wind power is estimated by considering the percentage of land area that is well exposed, moderately exposed, and poorly exposed in each land-surface form, as described in Appendix A. The areal distributions have been determined for each cell of a 1/4° latitude by 1/3° longitude grid in the contiguous U.S., 1/2° latitude by 1° longitude in Alaska, and 1/8° latitude by 1° longitude in Hawaii, Puerto Rico, and the Virgin Islands.
The areal distribution is shown in Maps 2-10 and 2-20 for grid cells in which the annual average wind power is class 3 or greater and in Maps 2-11 and 2-21 for power class 4 or greater. Grid cells where 80% or more of the total land area has class 4 power are mostly located in the southern and northern Great Plains, coastal areas of Texas, and scattered areas along the Northeast coast and Great Lakes. Throughout the Appalachians and mountainous areas in the West, high wind resource only exists on a small fraction (1 to 20%) of the land area. In many mountainous areas, only 2 to 5% of land area is estimated to be well exposed. The isolated grid cells scattered throughout parts of California, Oregon, Washington, and Montana where class 4 power occurs over more than 20% of the land area in the cell represent windy coastal strips or islands in the coastal areas and wind corridors in the inland areas (such as San Gorgonio Pass in California, the Columbia River and Ellensburg corridors in Oregon and Washington, and the Whitehall and Livingston corridors in Montana). Over 50% of the land area in much of southern and central Wyoming and the plains in northwestern Montana has class 4 or greater annual average wind power.

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