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It is often useful to know the maximum gust speed that can be expected to occur in any given time interval. This is usually represented by a gust factor G, which is the ratio of the gust wind speed to the hourly mean wind speed. G is obviously a function of the turbulence intensity, and it also clearly depends on the duration of the gust – thus the gust factor for a one‐second gust will be larger than for a three‐second gust, because every three‐second gust has within it a higher one‐second gust.

Figure 2.8 Gust factors calculated from Eq. (2.46)

While it is possible to derive expressions for gust factors starting from the turbulence spectrum (Greenway 1979; ESDU 1983), an empirical expression due to Weiringa (1973) is often used because it is much simpler and agrees well with theoretical results. Accordingly, the t‐second gust factor is given by

(2.46)

where I_u is the longitudinal turbulence intensity. Figure 2.8 shows the gust factors for several different turbulence intensities and gust durations calculated according to this expression.