The wind resource is expressed in terms of wind power classes, ranging from class 1 (the lowest) to class 7 (the highest). Each class represents a range of mean wind power density or approximate mean wind speed at specified heights above the ground (see power class legend). Areas designated class 3 or greater are suitable for most wind energy applications, whereas class 2 areas are marginal and class 1 areas are generally not suitable.
Local terrain features may interact with the windfield to cause the wind power to vary as much as ~ 50% to 100% from the assessment value. Thus, there may be local areas of high wind power in regions estimated to have low wind power; conversely, some local areas may have lower wind power than that shown by this assessment. Maps depicting the degree of certainty of these assessment values should be used in combination with this wind resource map.
The analyses of mean wind power apply to terrain features that are well exposed to the wind, such as plains, tablelands, hilltops, ridge crests, and mountain summits. In wooded or urban areas, the assessment values represent large clearings and other locations free of obstructions to the wind.
The physical characteristics of the land-surface form affect the number of wind turbines that can be sited in exposed places. For example, over 90% of the land area in a flat plain may be favorably exposed to the wind, whereas in mountainous terrain only the ridge crests (45% of the land area) may represent exposed places. On this map, where local relief generally exceeds 1000 ft. mountainous areas and prominent ridge crests are outlined in heavy, black lines with tick marks. Within these areas, wind resource estimates are for exposed ridge crests and mountain summits.