Wind — Physics

Wind is faster higher up, because the surface slows the bottom layer through friction. A gust is a pocket of that faster air from higher up getting mixed down to the water. More up-and-down mixing means gustier wind at the surface.

When the air is warmer than the water under it, the water cools the bottom layer of air. Cool air is heavier, so it stays put at the bottom and resists being stirred — like oil sitting on water. Fast air from above rarely reaches the surface, so the wind you feel is smooth with few gusts. Warm air over cool water is the textbook case.

The opposite case is cold air over warmer water, or over land heated by the sun. The bottom layer warms, gets lighter and rises, and faster air drops down to replace it. That churning gives punchy, gusty wind.

• ·Stable days: smooth and easy to ride, but the wind sometimes does not fully reach the surface. A 14 kn forecast can feel like 11 kn.
• ·Unstable days: the gust number matters more than the average. Size the kite for the gusts.

The wind speed in a forecast is the average over a short period; the gust is the peak burst within the hour. By definition the peak cannot sit below the average.

GFS computes average wind and gusts as two separate calculations. Its gust is roughly the average wind plus an allowance for how much stirring it expects in the air. In stable air it expects almost none, so the allowance is near zero — and because the two calculations are slightly out of sync, the gust can land a little under the average. For Rockaway in the first week of October, 30 of 168 hours came back that way (13.8 kn wind, 10.3 kn gusts).

Gust equal to (or just under) the wind means steady air with no real gusts on top. The /wind page now floors gusts at the wind speed so the number reads honestly.