UK Met Office Handbook says:
Table 6.4(a). Ratio of maximum (3-second) gust to mean hourly speed (for
strong, steady 10 m winds)
Surface type, Range of Estimated
ratios, average
Open sea 1.3, 1.3
Isolated hill tops 1.4-1.5, 1.4
Flat open country 1.4-1.8, 1.6
*Rolling country (few wind-breaks) 1.5-2.0, 1.7
Rolling country (numerous wind-breaks),
forest areas, towns, outskirts of large cities 1.7-2.1, 1.9
Centres of large cities 1.9-2.3, 2.1
Local variations, using this commonly used category, often give gusts varying widely in space and time from the estimated values, making airfield forecasting difficult, especially under isallobaric surging.