Wind Roses

From Wind wiki

Wind roses are used to present wind speed data and wind direction data that has been collected over some time, so that the dominant wind pattern for a particular area can be determined. Wind roses are also useful as they project a large quantity of data in a simple graphical plot.

Presented in a circular format, the wind rose shows the frequency of winds blowing from particular directions over a thirty year period. The length of each "spoke" around the circle is related to the frequency that the wind blows from a particular direction per unit time. Each concentric circle represents a different frequency, emanating from zero at the center to increasing frequencies at the outer circles. A wind rose plot may contain additional information, in that each spoke is broken down into color-coded bands that show wind speed ranges. Wind roses typically use 16 cardinal directions, such as north (N), NNE, NE, etc., although they may be subdivided into as many as 32 directions. The length of each "arm" is proportional to the fractional frequency at which that wind speed (and below) was observed from that direction. Different colors on each "arm" indicate the wind speed.

Wind roses from neighboring 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.