WAsP uses the 2-parameter Weibull distribution to represent wind speed distributions, directly observed wind speeds organized in one histogram per sector, and predicted wind speed distributions, also per sector.

The Weibull function is defined as

where A [m/s] is the scale parameter and k is the shape parameter, the larger the k-value, the narrower the distribution.

The corresponding cumulated distribution function is given by

The fitting to an observed histogram is performed by the following two requirements:

a: M₃(f_Weibull) = M₃(histogram) (essentially matching of the power density)

b: F_Weibull(U_char) = F_histogram(U_char)

where M_n indicates the n'th moment of a distribution and F_histogram(U) indicates the cumulated probability of the histogram from 0 to U, assuming the histogram to be normalized to 1. U_char is a characteristic wind speed of the distribution, selected as the mean speed of the histogram distribution:

U_char = M₁( histogram )

This fitting procedure has been developed with special regard to wind power applications since it ensures a good fit at higher wind speeds, whereas less weight is put on agreement between fitting function and histogram at low wind speeds since the wind energy here is insignificant anyway.