The atmosphere is a great mass of fluid, gliding over the surface of the Earth. When the terrain is relatively smooth, so there is little turbulence, each layer of air stays at its own level; each also exerts a frictional force on the layers above and below it, tending either to slow them down or to speed them up, respectively, and the ground itself exerts a frictional drag on the very lowest layer of air.
The friction of the ground, indeed, is sufficient to bring the air immediately adjacent to it to a standstill. Some hundreds of feet up, on the other hand, above what we call the "friction layer", the wind speed is that dictated by the spacing of the isobars. In between, were it not for the complicating effects of turbulence, wind would gradually increase with height from near zero at the ground to the speed of the "free wind" some hundreds of feet above.
In reality, however, there is always some element of turbulence. The roughness of the terrain affects the wind in much the same way as rocks on a riverbed disturb the flow of water. If the ground is relatively smooth, or over a calm sea, the flow will be steady and undisturbed. But over an uneven surface, eddies form, and are carried away by the wind, becoming apparent as gustiness downstream. Turbulence tends to increase the wind speed near the ground, and by doing so it decreases the tendency for marked variations with height within the friction layer.
But although for this reason the assumption that wind increases gradually with height does not apply exactly, it is true in a general way. To avoid any confusion, therefore, meteorologists always measure the wind at a height of exactly 10 metres, or 33 ft, above ground level. By using this standard height, they can be sure that they are always comparing like with like when wind data from anywhere in the world are being analysed. And this is also the height they refer to when giving forecasts of expected wind strength.
Detailed studies have been carried out as to how one might calculate the wind strength above and below this standard level, and Hellman's Formula is taken to be the best guide. This formula is rather complex, but broadly speaking it suggests that at a height of 10 feet, the wind is something like 20 per cent less than the 33 feet value; 100 feet above the ground, it is around 20 per cent greater; and 200 feet above ground level, it may be up to 50 per cent higher than that measured at the standard height.