Geostrophic Balance vs Surface Drag – The 15% Wind Drop from 100 m to 2 m
You check the wind forecast. It predicts 20 knots of geostrophic wind (the wind flowing freely in the upper atmosphere). You hit the water, but your handheld meter only reads 14 knots. You feel cheated.
You are experiencing the "Friction Drop." The wind at 100 meters altitude is in Geostrophic Balance—a perfect tug-of-war between the Pressure Gradient Force and the Coriolis Effect. But at the surface (2 meters), friction destroys this balance.
The Logarithmic Profile
Wind does not slow down linearly as it gets closer to the water. It follows a "logarithmic profile".
- 100 meters: The wind flows at 100% speed (e.g., 20 knots).
- 10 meters: The standard measurement height for weather stations. Friction reduces speed by ~10–15%.
- 2 meters: The height of your wing. Friction reduces speed by another ~15%.
This means the wingfoil wind you actually feel is often 30% weaker than the gradient wind shown on broad-scale GFS charts.
The Water Drag Coefficient
Water is "sticky." As wind blows over it, it grabs the surface to create swell and chop. This transfer of energy acts as a brake on the air.
In light wind (8–12 knots), the water is flat. The drag is low. The drop from 10m to 2m is small. In high wind (25+ knots), the water is rough. The drag increases (higher roughness length $z_0$). The "brake" is applied harder. The difference between the wind at your mast tip and your board becomes extreme.
The "Crossing Angle"
Friction does not just slow the wind; it turns it. Because the wind slows down, the Coriolis force weakens. The Pressure Gradient Force takes over and pulls the wind toward the Low pressure.
- Result: The surface wind blows across the isobars at an angle of about 15–20 degrees over water.
- Impact: If you look at a pressure map, do not assume the wind direction is perfectly parallel to the lines. Add a 20-degree twist toward the Low.
Summary
When you see a wind forecast for 20 knots, assume it is measured at 10 meters. Expect only 15–17 knots at board height. Rig your wingfoil wind gear bigger to compensate for the friction layer.