Apparent wind

Apparent wind is the wind experienced by a moving object.

The Apparent wind is the wind experienced by an observer in motion and is the relative velocity of the wind in relation to the observer.

Apparent wind velocity is the vector sum of the true wind and the headwind an object would experience in still air. The headwind velocity in still air is inverse of the object's velocity, therefore the apparent wind can also be defined as a vector subtraction: the Velocity of the wind minus the Velocity of the object.

In sailing, the apparent wind is the actual flow of air acting upon a sail. It is the wind as it appears to the sailor on a moving vessel. It differs in speed and direction from the true wind that is experienced by a stationary observer. In nautical terminology, these properties of the apparent wind are normally expressed in knots and degrees. On boats, apparent wind is measured (see "Instruments" below) or "felt on face / skin" if on a dinghy or looking at any telltales or wind indicators on board. True wind needs to be calculated or stop the boat.

Note that a number of additional factors come into play when converting the measurements from the masthead anemometer into the true wind if a high degree of accuracy is required, including the following:

Whilst in non-tidal waters it is valid to define the true wind as the wind when the vessel is stationary, but where there is a water flow (whether from tides or sailing on a river) this also has an effect on the wind experienced, and this is independent of the motion of the boat through the water. This is commonly factored out by defining the true wind as the wind experienced when the boat is drifting with the water (but moving with respect to the sea bed), and then defining the wind when the boat is stationary with respect to the sea bed as the ground or geographical wind.

The apparent wind on board (a boat) is often quoted as a speed measured by a masthead transducer containing an anemometer and wind vane that measures wind speed in knots and wind direction in degrees relative to the heading of the boat. Modern instrumentation can calculate the true wind velocity when the apparent wind and boat velocity and direction are input.

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