Wind Rose

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 Wind formation along a) the coast lines, and b) mountain ranges
Figure 1: Wind formation along a) the coast lines, and b) mountain ranges

Selection of a proper site for installing wind farms requires detailed meteorological data at different times. Wind data are routinely collected by wind anemometers installed on top of towers in the direction of the prevailing wind. Because speeds and directions are continually changing, 10-15 minute average values are recorded. The data is plotted on a wind rose, which is a single, graphical representation of speed, direction, and frequency of occurrence. Data are often normalized to the total period of observation in order to indicate frequencies. One simple way to graphically indicate both duration and direction is to draw bars extending radially from the centre of the rose in the direction of the wind; magnitude is represented proportionally to the time the wind spent in that direction at a given speed. The direction of the wind is traditionally taken as the direction it is coming from. The thickness or the color of the bars can be used to indicate the range of speeds in a given direction. A typical wind rose is shown in Figure 2. The data were accumulated over a period of ten years in Fresno, California, for the month of April. From the figure, it can be deduced that over 50% of the time the wind blows to the northwest at speeds of 1.8-3.3 m/s (16% of the time), 3.3-5.4 m/s (20% of the time), 5.4-8.5 m/s 11% of the time, 8.5-11 m/s 2% of the time and greater than 11 m/s less than 1% of the time.

Figure 2: Wind rose for Fresno, California for the month of April. Concentric circles represent the frequency, and different colors indicate ranges of wind velocity.

 United States Wind Resource Map
Figure 3: United States Wind Resource Map

Depending on the wind power density and speed, different geographical areas have been divided into seven classes. Class 1 regions are not suitable for wind energy development, and class 2 regions may become only marginally acceptable. Class 3 areas will be suitable in the future, as technology matures. Class 4 and higher are considered suitable for wind power with existing technology.

Figure 3 shows the yearly electricity production in W/m2 of rotor area for different locations within the continental United States. As the data indicates, the strongest winds are found along coastlines, along ridges, and on the Great Plains. (1, 2) Furthermore, it is estimated that up to 6% of US land is suitable for development of wind farms and has the potential to generate 1.5 times its electricity needs. (3)


(1) Wind Energy Resources of United States, U.S. Department of Energy (DOE), can be found at the National Wind Technology Center web site at

(2) DOE “Wind Powering America,” web site at

(3) US Department of Energy (

Further Reading

Gipe P., Wind Energy Basics, —A comprehensive guide to modern small wind technology. AWEA (

Elliott, D. et al., Wind Energy Resource Atlas of the United States, by American Wind Energy Association (

Khennas, S., Small wind systems for rural energy services, London: ITDG Pub., 2003.

Solar Energy, Direct Science Elsevier Publishing Company, the official journal of the International Solar Energy Society ®, is devoted to the science and technology of solar energy applications, and includes the indirect uses such as wind energy and biomass.

Home Power Magazine—bimonthly magazine for farm and home wind turbines (

External Links

Energy Efficiency and Renewable Energy Clearinghouse (

National Wind Technology Center, National Renewable Energy Laboratory (

Energy Efficiency and Renewable Energy: Wind Energy Technologies, US DOE (

American Wind Energy Association (