Editing Apparent wind (section)

==Apparent wind in sailing==
In [[sailing]], ''apparent wind'' is the speed and direction of wind indicated by a wind instrument ([[anemometer]]) on a ''moving'' craft (on water, land or ice) in undisturbed air. It is composed of the ''combined'' speeds and directions of the craft and wind observed by a ''stationary'' wind instrument—the ''true wind''. A true wind coming from the bow increases the apparent wind induced by the speed of the craft, coming from the stern it decreases apparent wind, and coming from the side the apparent wind angle and speed change according to the combined speed and direction of each the craft and the true wind. Apparent wind is important to sailors in order to set sail angle with respect to the wind and to anticipate how much power the wind will generate on a [[point of sail]]. Apparent wind differs in speed and direction from the '''true wind''' that is experienced by a stationary observer and composed of the true wind speed (TWS) and true wind direction (TWD) or the TWS and true wind angle (TWA) relative to the boat if it were stationary.<ref>{{cite web|title=What are My Electronics Telling Me About Boatspeed and Heading?|url=https://www.sailingworld.com/what-are-my-electronics-telling-me-about-boatspeed-and-heading|website=Sailing World|date=21 May 2015 |accessdate=22 October 2017}}</ref>
In [[nautical terminology]], apparent wind is measured in [[Knot (unit)|knots]] and [[degree (angle)|degree]]s.

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:<ref>{{cite book|last1=Thornton|first1=Tim|title=The Offshore Yacht|publisher=Adlard Coles}}</ref><ref>{{cite book|last1=Marchaj|first1=C.A.|title=The AeroHydrodynamics of Sailing|publisher=Adlard Coles}}</ref><ref>{{cite web|title=Sailing Instruments Calibration|url=http://www.ockam.com/category/sailing-instruments-calibration/|website=Ockam Instruments|accessdate=10 June 2015}}</ref>
* [[Leeway]] (or drift on power vessels) - Factors like water currents or slipping sideways due to wind (leeway) mean that the direction a craft is pointing often does not exactly match its actual direction of travel. This must be corrected for when converting apparent wind angle to true wind direction. The same effect is found when the craft is altering course.
* Mast twist - the rigging loads often put a significant amount of torsion on the mast, especially if the rig has runners, so it is twisted along its length
* Mast rotation - many racing [[multihull]]s have a mast that can be rotated, so the anemometer reading needs to be corrected by the angle of rotation of the mast
* Heel angle - this is a simple trigonometric correction
* Upwash from the sails - the airflow around the top of the mast is distorted by the presence of the sails. This effect varies with the sails set at the time, the wind speed and the point of sail, but is noticed by the true wind angle changing from port to starboard tack, and the true wind speed changing from when beating to running
* Boat motions - as the masthead is so distant from the centre of motion of the craft, inertial effect on both the wind vane and the anemometer cups can be significant when the craft is moving, especially when pitching and rolling
* Wind shear - there can be a significant change in both wind speed and direction between the water's surface and the top of the mast, especially in conditions of unstable, light airs. The wind instruments are just measuring conditions at the masthead, and these are not necessarily the same at all heights

In the presence of a current, the true wind is considered to be that measured on the craft drifting with the water over the bottom, and wind with respect to the sea bed as the ''ground'' or ''geographical wind''.{{cn|date=September 2016}}