WIND ROSE

Wind rose: the wind data direction duration and intensity are graphically represented by a diagram called wind rose. The wind data should usually be collected for a period of at least 5 years and preferably of 10 years so as to obtain an average data with sufficient accuracy.
Wind rose diagrams can be plotted in two types
1. showing direction and duration of wind
2. Showing direction duration and intensity of wind.
Type – I: This type of wind rose is illustrated in fig. the radial lines indicate the wind direction and each circle represents the duration of wind. The values are plotted along the north direction in fig similarly other values are also plotted along the respective directions. All plotted points are then joined by straight lines. The best direction of runway is usually along the direction of the longest lone on wind rose diagram. If deviation of wind direction up to 22.5º + 11.25ºfrom their direction of runway is thus along NS direction of landing and take off is permissible the percentage of time in a year during which runway can safely be used for landing and take off will be obtained by summing the percentages of time along NNW, N, NNE, SSE, S and SSW directions. This comes to 57.6 percent. The total percentage of the time therefore comes to 57.0 + 13.5 = 70.5. This type of wind rose does not account for the effect of cross wind component.

Type – II : this type of wind rose is illustrated in fig. the wind data as in the previous type is used for this case. Each circle represents the wind intensity to some scale. The values entered in each segment represent the percentage of time in a year during which the wind having a particular intensity blows from the respective direction. The procedure for determining the orientation of runway from this type of wind rose is described below.
Draw three equi spaced parallel lines on a transparent paper strip in such a way that the distance between the two near by parallel lines is equal to the permissible cross wind component. This distance is measured with the same scale with which the wind rose diagram is drawn the permissible cross wind component is 25kph. Place the transparent paper strip over the wind rose diagram in such a way that the central line passes through the centre of the diagram. With the centre of wind rose rotate the tracing paper and place it in such a position that the sum of all the values indicating the duration of wind within the two outer parallel lines is the maximum. The runway should be thus oriented along the direction indicated by the central line. The wind coverage can be calculated by summing up all the percentages.