To represent of Annual and Seasonal Variations we can use The Weibull distribution, The Weibull distribution has been found to give a good representation of the variation in hourly mean wind speed over a year at many typical sites. This distribution takes the form

where F(U) is the fraction of time for which the hourly mean wind speed exceeds U.

because k describes the variability about the mean, and c is related to the annual mean wind speed U, they have another relation like this formula

which I(at those symbol like F or I) is the complete gamma function

since the mean wind speed is given by

those relation can be derived by consideration of the probability density function