Gust factors over open water and built-up country

Data are presented of the gust factorG =umax/ū, both at 8-m height over a lake and up to 80-m height at the edge of a town. It is argued that, due to the presence of trends in long-period averages, the use of arithmetic or least-squareG-averages leads to overestimation of the dependence ofG on wind speedu, and medianG-values are preferable. A simple non-spectral model for gustiness at high wind speeds in the constant-stress layer is proposed and checked. The model relatesG to surface roughness and height above surface for gust wavelengths up to ≈ 200 m. The gust factor concept is shown to be inadequate for characterizing thunderstorm gustiness.