The Wind Rose
Wind rose from Brest, France, taken from the European Wind Atlas, Riso National Laboratory, Denmark.
Wind Rose, Brest, France You will notice that strong winds usually come from a particular direction, as discussed in the Wind Energy Resource section.
To show the information about the distributions of wind speeds, and the frequency of the varying wind directions, one may draw a so-called wind rose on the basis of meteorological observations of wind speeds and wind directions.
The picture shows the wind rose for Brest, on the Atlantic coast of France.
We have divided the compass into 12 sectors, one for each 30 degrees of the horizon. (A wind rose may also be drawn for 8 or 16 sectors, but 12 sectors tend to be the standard set by the European Wind Atlas, from which this image was taken).
The radius of the 12 outermost, wide wedges gives the relative frequency of each of the 12 wind directions, i.e. how many per cent of the time is the wind blowing from that direction.
The second wedge gives the same information, but multiplied by the average wind speed in each particular direction. The result is then normalised to add up to 100 per cent. This tells you how much each sector contributes to the average wind speed at our particular location.
The innermost (red) wedge gives the same information as the first, but multiplied by the cube of the wind speed in each particular location. The result is then normalised to add up to 100 per cent. This tells you how much each sector contributes to the energy content of the wind at our particular location.
Remember, that the energy content of the wind varies with the cube of the wind speed, as we discussed in the page on The Energy in the Wind. So the red wedges are really the most interesting ones. They tell us where to find the most power to drive our wind turbines.
In this case we can see that the prevailing wind direction is Southwest, just as we would have predicted from the page on Global Winds.
A wind rose gives you information on the relative wind speeds in different directions, i.e.each of the three sets of data (frequency, mean wind speed, and mean cube of wind speed) has been multiplied by a number which ensures that the largest wedge in the set exactly matches the radius of the outermost circle in the diagram.
Wind Roses Vary
Wind rose from Caen, France Wind roses vary from one location to the next. They actually are a form of meteorological fingerprint.
As an example, take a look at this wind rose from Caen, France, only about 150 km (100 miles) North of Brest. Although the primary wind direction is the same, Southwest, you will notice that practically all of the wind energy comes from West and Southwest, so on this site we need not concern ourselves very much about other wind directions.
Wind roses from neighbouring areas are often fairly similar, so in practice it may sometimes be safe to interpolate (take an average) of the wind roses from surrounding observations. If you have complex terrain, i.e. mountains and valleys running in different directions, or coastlines facing in different directions, it is generally not safe to make simple assumptions like these.
The wind rose, once again, only tells you the relative distribution of wind directions, not the actual level of the mean wind speed.
How to Use the Wind Rose
A look at the wind rose is extremely useful for siting wind turbines. If a large share of the energy in the wind comes from a particular direction, then you will want to have as few obstacles as possible, and as smooth a terrain as possible in that direction, when you place wind turbines in the landscape.
In these examples most of the energy comes from the Southwest. We therefore need not be very concerned about obstacles to the East or Southeast of wind turbines, since practically no wind energy would come from those directions.
You should note, however, that wind patterns may vary from year to year, and the energy content may vary (typically by some ten per cent) from year to year, so it is best to have observations from several years to make a credible average. Planners of large wind parks will usually rely on one year of local measurements, and then use long-term meteorological observations from nearby weather stations to adjust their measurements to obtain a reliable long term average.
Since this wind rose comes from the European Wind Atlas we are reasonably confident that we can rely on it. The European Wind Atlas contains a description of each of the measurement stations, so we may be warned about possible local disturbances to the airflow. On the page on selecting a wind turbine site, we return to the pitfalls in using meteorology data.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 19 September 2003
http://www.windpower.org/en/tour/wres/rose.htm
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