How to interpret a wind rose?

 

A wind rose gives a very succinct but information-laden view of how wind speed and direction are typically distributed at a particular location. Presented in a circular format, the wind rose shows the frequency of winds blowing FROM particular directions. The length of each "spoke" around the circle is related to the frequency of time that the wind blows from a particular direction. Each concentric circle represents a different frequency, emanating from zero at the center to increasing frequencies at the outer circles. The wind roses shown here contain additional information, in that each spoke is broken down into discrete frequency categories that show the percentage of time that winds blow from a particular direction and at certain speed ranges. All wind roses shown here use 16 cardinal directions, such as north (N), NNE, NE, etc.

An example is shown here. It is the wind rose for Chennai, India based on three months of hourly wind data (all hours of the day). This rose shows that the winds at Chennai during the period blow from the southeast much of the time. In fact, the 3 spokes around the southeast direction (ESE, SE and SSE) comprise 39% of all hourly wind directions. This is quickly calculated by taking the sum of the frequencies of each of these directions (13+16+10=39%). This also shows that the wind rarely blows from the northwest. These wind roses also provide details on speeds from different directions. Examining winds from the southeast (the longest spoke) one can determine that approximately 1% of the time the wind blows from the southeast at speeds between 3.6 and 7.2 kilometer per hour. Similarly, on this spoke it can be calculated that winds blow from the southeast at speeds between 7.2 and 10.8 km/hr about 3% of the time, at speeds between 10.8 and 18 km/hr about 7% of the time, between 18 and 28 km/hr about 5% of the time.

Please note the legend at the bottom of the wind rose that gives the speed categories and their associated colors.

The legend at the bottom gives additional information such as the unit (km/hr), the average wind speed for the overall hours (in this case 12.8 km/hr), and percentage of time that the winds are calm (0%), and the years and month and hours of data on which each rose was constructed.

To calculate the typical amount of time that the wind blows from a particular direction and certain speeds just multiply the respective frequency by the appropriate amount of time.