GWD, TWA, AWA: Wind in all its forms

Wind: definitions

GWD / GWS

Ground Wind Direction / Ground Wind Speed

This is the direction and speed of the wind in the ground (earth) reference frame. It is the wind from forecasts provided by weather services, received in GRIB files (average wind at 10 metres).

AWA / AWS

Apparent Wind Angle / Apparent Wind Speed

This is the angle and speed of the apparent wind in the boat's sails. The measurements are provided by the boat's wind vane/anemometer sensor. The apparent wind is the instantaneous result of the ground wind combined with the wind created by drift — drift due to wind and drift due to current — and by the boat's speed through water.

TWA / TWS

True Wind Angle / True Wind Speed

This is the angle and speed of the true wind (real wind) received by the boat in the water surface reference frame. It is computed from the apparent wind, corrected for the wind caused by drift (wind + current) and the wind caused by the boat's speed through the water.

Wind: illustration

The transposition between the two diagrams below, from left to right, highlights the apparent wind and true wind angles relative to the boat's course over water.

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