Wind Gust to Mean Ratio

Mean wind vs. gust

A gust is the instantaneous wind speed held for roughly 3 seconds. The mean wind, by contrast, is averaged over 10 minutes following the WMO standard. The gust factor G = V_gust / V_mean usually lands between 1.5 and 2.0 over rough terrain, dropping to somewhere around 1.2 to 1.4 over open water. Run the numbers and a 60 km/h gust with G = 1.4 works out to a 10-minute mean near 43 km/h. As for the Beaufort scale, it runs from 0 (calm) up to 12 (hurricane, above 117 km/h), and Saffir-Simpson Category 5 hurricanes push past 252 km/h sustained. Brazil's INMET reports the maximum gust alongside the mean velocity in METAR and SYNOP.

Applications

In aviation, ATIS and METAR report mean wind plus gusts so crews can judge runway safety. At sea, gusts running above the mean are the cue to reef the sails. Civil engineers lean on the Brazilian NBR 6123 standard, which adopts the 3-second gust as the design wind. Formula 1 weather radar tracks gust direction at every Grand Prix, and wind turbines pitch their blades or shut down once gusts cross a set threshold.

FAQ

Why use 3-second gust and 10-minute mean? Three seconds roughly matches how fast typical anemometers and structures respond. Ten minutes is long enough to average out the turbulence but still short enough to pick up shifts in synoptic conditions.

Why is G higher on land than at sea? Buildings, trees and hills roughen the surface and stir up turbulence, which pushes the peak gusts up relative to the mean. Over open water and flat terrain the flow stays smoother, and G settles near 1.2.

Does the gust factor depend on speed? A little. At very low wind speeds the gust factors get noisy, but once you reach design winds above 50 km/h, G tends to settle around the value typical for that terrain.