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Wind Data Links
When no wind speed measurements are available for a wind turbine site, published wind data is certainly a good place to start. Here are a number of web sites that provide wind data for the United States.
NCDC Wind data for various cities
Monthly average windspeed for many cities in the United States.
NOAA Climatic data for various cities
Monthly average windspeed, peak windspeed, and primary direction for many cities in the United States.
(NREL) United States Wind Energy Resource Atlas
A very extensive survey of wind availability in the United States.
NRCS National Weather and Climate Center
Provides wind data in the form of a "wind rose" which provides speed and direction as well as percent time at that speed and direction.
Help! The wind data is for a 50 meter tower!
Yes, wind data is often provided at some height above the reach of the average wind power enthusiast. There is a way to estimate the wind speed at a lower elevation; however, the farther you get from the height where the wind speed was measured, the less reliable the estimate. As one gets closer to the ground, the wind will be more affected by buildings, trees, hills etc. so remember, they are just ESTIMATES. The "wind roses" are helpful too. For instance, our wind often comes from the south, but we have a clump of trees to the south so during those times our wind is low.
A long time ago, a man named Prandtl suggested that velocity of fully developed turbulent fluid flow over a flat plate could be represented by a function to the seventh root. We can adopt this relationship for our purposes like so:
A long time ago, a man named Prandtl suggested that velocity of fully developed turbulent fluid flow over a flat plate could be represented by a function to the seventh root. We can adopt this relationship for our purposes like so:
The wind speed around us is about 6.8 m/s at 50 meters. But my tower is only at a height of 3 meters. So one could estimate that the wind speed at my tower would be more like:
One can improve on the "seventh root" function by taking measurements over various types of terrain and determining a better exponent to model the wind speed. A more sophisticated approach can be found here:
A more sophisticated set of exponents
What does the average wind speed really mean?
Of course, the wind does not always blow and once in a while it howls. So the designer has to use statistics to describe the wind. We know that the wind speed cannot go below zero and it is very unlikely to be very high. One way to describe the wind is to use a "Rayleigh Distribution" which is a special case of the "Weibull Distribution." The Rayleigh distribution looks like this for an average wind speed of 4.5 m/s. If you would like curves for other wind speeds, see the attached spreadsheet. One property of probability density functions is the area under the curve is equal to 1.00, or one could think of it as 100%. If one wants to find the probability of wind being between one speed and another, just measure the area under the curve between those speeds.
Measuring the area under the curve isn't always easy, to help out, the Rayleigh distribution can be expressed in the form of a "Cumulative Distribution" curve which gives the area under the density curve up to that speed. The cumulative distribution curve for an average wind speed of 4.5 m/s looks like this:
Using this curve, it is easy to see what the area is under the density function curve up to any wind speed. For instance, the area to the left of 6.0 m/s is about 0.75 or 75%. There is a 75% chance the wind will be less than 6.0 m/s.
rayleighdist.xlsx  
File Size:  20 kb 
File Type:  xlsx 
Example 1: My average wind speed is 4.5 m/s. What is the probability that the windspeed will be less than 3.0 m/s?
With questions like this, we are interested in the area to the left of 3.0 m/s in the probability density function. To find the area, we just turn to the cumulative distribution curve, locate 3.0 m/s and read the value 0.295. That means there is a 29.5% chance the wind will be less than 3.0 m/s. Remeber this is just an estimate, the real value is something a bit different.
Example 2: My average wind speed is 4.5 m/s. What is the probability that the wind speed will be between 3.0 m/s and 5.0 m/s?
With questions like this, we are interested in the difference between the area to the left of 5.0 m/s and the area to the left of 3.0 m/s (the area in green). To get this, we take the cumulative value 0.621 for 5.0 m/s and subtract the 0.295 we found earlier for 3.0 m/s and get 0.326 or a 32.6% chance the wind will be in that range. Again ... just an estimate.
Weibull distribution reference
Rayleigh distribution reference
Please Note: Information displayed on this web site in text and/or video form is for documentation purposes only. Nothing presented here should be considered professional advice or instruction. The visitor should follow safe procedures in their own projects and check all information for accuracy. Caleb Engineering, LLC is not responsible for any losses that may result from errors, misprints or display of improper procedure.