WIND RESOURCE

Wind rose

Introduction
Strong winds usually come from a particular direction, as discussed already. To show the information about the distributions of wind speeds and the frequency of the varying wind directions the wind rose may be used. It comes on the basis of meteorological observations of wind speeds and wind directions.
The image shows the wind rose for Brest, on the Atlantic coast of France, as was taken from the European Wind Atlas. The compass has been divide 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. The following different wedges can be distinguished:
- 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%. This tells 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%. This tells how much each sector contributes to the energy content of the wind at our particular location. The energy content of the wind varies with the cube of the wind speed, as will be discussed later. So the red wedges are really the most interesting ones since they tell where to find the most power to drive wind turbines.
In this case it can be seen that the prevailing wind direction is Southwest, just as it could be predicted from the page on Global Winds.

Other wind rose forms
A wind rose gives 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 can be depicted in several different ways. This figure is a different layout of a windrose, for the wind conditions in the Netherlands. The wind rose shows how often the wind comes from a certain direction in July for the time period 1971-2000. The directions have been split up into 12 segments of 30 degrees. The longer a bar, the more often the wind comes from the corresponding direction segment. The wind rose presents a scale which can be used to read the relative frequency. All bars including the variable winds and calms (calm/var) add up to 100%. The width of the bars gives an indication of the wind speed. Three classes are used: 1-2 Beaufort, 3-4 Beaufort and 5 Beaufort and more.

Wind roses vary
Wind roses vary from one location to another. They actually are a form of meteorological fingerprint. As an example, look at this wind rose from Caen, France, only about 150 km North of Brest. Although the primary wind direction is the same, southwest, here practically all of the wind energy comes from west and southwest, so on this site the other wind directions are not of big importance.
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. However, for complex terrains, 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 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 it is preferable to have as few obstacles as possible and as smooth a terrain as possible in that direction, for placing wind turbines in the landscape. In the previous examples most of the energy comes from the southwest. Thus, not big concern should be given about obstacles to the east or southeast of wind turbines, since practically no wind energy would come from those directions.
However, note that wind patterns may vary from year to year and the energy content may vary, typically by about 10%, 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 it can be considered reliable. 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 Selecting a site, notes on pitfalls in using meteorology data are given.

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