The fundamental scaling velocity, equal to the square root of the surface stress, to, divided by the air density. The surface stress can be observed by special instruments that directly observe the drag at the surface, or by fast response turbulence instruments using the definition: to = r(-u'w') = ru*2 where u' is the longitudinal wind speed fluctuation, w' is the vertical wind speed fluctuation, and the average is over about a one-hour time period. The variable u* can also be estimated from wind observations. A rough rule of thumb is that the ratio u*/u is about 0.05 to 0.1, where u is the wind speed at a height of about 10 m, which is the standard measurement height at airports around the world. u* has typical values ranging from about 0.05 m/s in light winds to about 1 m/s in strong winds.
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