Wind: mean, gusts and coasts

What the wind number is

• Height. The standard forecast wind is at 10 m above the surface. Wind is stronger higher up, so the wind at your masthead is a little more than the forecast value, and the wind you feel in the cockpit is less.
• Averaging. The international standard for wind observations is a ten-minute mean. A model’s wind is a similar smoothed value that represents a whole grid box.
• Mean wind versus gusts. A gust is a brief peak, usually averaged over about three seconds. A forecast gust is not measured: the model estimates it from the mean wind, the turbulence near the surface and how stable the air is. Over open sea, gusts are commonly 30 to 60% above the mean wind; over land and in showery weather the difference is larger.
• Apparent wind. Instruments on a moving boat measure the apparent wind. Subtract your own motion to compare it with a forecast.

Why the coast is where forecasts go wrong

Over open water the wind is smooth and large-scale, and a model can describe it well. Near land, several things happen at scales of a few to a few tens of kilometres, smaller than the box of a coarse model.

• Land friction. The wind slows and turns as it crosses from sea to land and back. A single box at the coast mixes both, so the model wind there is neither the sea value nor the land value.
• Sea breeze. On sunny days, land warms faster than the sea and draws air inland, adding to the wind in the afternoon. Models of about 4 km or finer start to show it; coarser ones often miss it or time it wrongly.
• Headlands and gaps. Wind accelerates around capes and through narrow channels or between islands, sometimes by a force or more.
• Mountains and valleys. Wind is channelled along valleys and down slopes and can spill out over the water as strong local winds. Smoothed terrain in the model weakens these.
• Showers and thunderstorms. Downdrafts spread out as sudden gusts that no coarse model can place correctly.