When sailing or learning the theory of sailing, you may have heard someone talking about true and apparent wind. This might be with regard to wind speed or wind direction.
True wind speed, sometimes known as ground wind, is the actual speed of the wind as it passes over land or the surface of the sea, assuming no tidal flow.
Apparent wind speed is the wind you ‘feel’ on you as you sail.
For example, were you stationary, at anchor, and feel a 10 kt breeze on your face, that would be the true wind speed but it would also be the apparent wind speed. They would be the same. However, were the Harbour Master to come along, pick up your anchor and start towing you at a speed over ground of 6kts, directly into the same 10kts breeze, you would now feel a 16kt apparent wind on your face. It would seem considerably breezier.
Were this deranged Harbour Master to then stop and drop your anchor again, and the true wind speed was still 10 kts, both true and apparent wind would again return to 10 kts, once you had stopped.
It’s important to know the difference between true and apparent wind for a variety of reasons. The true wind speed and direction, quoted in weather forecasts, will determine sea state. But when sailing your vessel you will be trimming and setting sail plans based upon the apparent wind. For example, you can safely hold more sail down wind in a Force 6 (say 25kts) because, if travelling at 10kts speed over the ground, the wind hitting your sails is the equivalent of just 15kts (25kts - 10kts), or a Force 4.
But, were you to change direction, come up on the wind and start to motor-sail dead into the wind at 6 kts, all of a sudden the apparent wind would feel like 31kts (a near gale!). The apparent wind could therefore vary between a pleasant F4 to an extremely unpleasant F7 and near gale, purely dependent on your direction of travel relative to the wind.
True wind speed will help you forecast sea state. But your direction of travel will help determine how you will experience this. Furthermore, where wind blows against tide, there is, again, a higher apparent wind on the sea’s surface, creating a worse sea state until the tide changes or the wind shifts. When passage planning we consider the true wind forecast and how that will affect sea state. We then sail the vessel by reference to the apparent wind speed and our desired course and direction of travel.
Modern instruments are quickly able to measure apparent wind speed and, by a simple calculation based upon the vessel’s speed and direction they will instantly inform us of the true and apparent wind speed, millisecond by millisecond. However, it’s important to make sure your instruments are calibrated and regularly checked, otherwise the information you will receive will be of little use.