Editing Sailing (section)

====Wind variation with height and time====
{{Further|Wind gradient#Sailing}}
Wind speed increases with height above the surface; at the same time, wind speed may vary over short periods of time as gusts.

[[Wind shear]] affects sailing craft in motion by presenting a different wind speed and direction at different heights along the [[mast (sailing)|mast]]. Wind shear occurs because of friction above a water surface slowing the flow of air.<ref>
{{Citation
 | last1 = Deacon
 | first1 = E. L.
 | last2 = Sheppard
 | first2 = P. A.
 | last3 = Webb
 | first3 = E. K.
 | title = Wind Profiles over the Sea and the Drag at the Sea Surface
 | journal = Australian Journal of Physics
 | volume = 9
 | issue = 4
 | pages = 511
 | date = December 1956
 | doi = 10.1071/PH560511 | bibcode = 1956AuJPh...9..511D
 | doi-access = free
 }}
</ref> The ratio of wind at the surface to wind at a height above the surface varies by a power law with an exponent of 0.11-0.13 over the ocean. This means that a {{cvt|5|m/s|kn|adj=on}} wind at 3 m above the water would be approximately {{cvt|6|m/s|kn}} at {{cvt|15|m|ft|sigfig=1}} above the water. In hurricane-force winds with {{cvt|40|m/s|kn}} at the surface the speed at {{cvt|15|m|ft|sigfig=1}} would be {{cvt|49|m/s|kn}}<ref>
{{cite web
 |last        = Hsu
 |first       = S. A.
 |title       = Measurements of Overwater Gust Factor From NDBC Buoys During Hurricanes
 |publisher   = Louisiana State University
 |date        = January 2006
 |url         = http://www.nwas.org/ej/pdf/2006-EJ2.pdf
 |access-date  = 19 March 2015
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20160304190331/http://www.nwas.org/ej/pdf/2006-EJ2.pdf
 |archive-date = 4 March 2016
}}
</ref> This suggests that sails that reach higher above the surface can be subject to stronger wind forces that move the centre of effort on them higher above the surface and increase the heeling moment. Additionally, apparent wind direction moves aft with height above water, which may necessitate a corresponding [[Sail twist|twist in the shape of the sail]] to achieve attached flow with height.<ref>
{{Citation
 | first1 = A.
 | last1 = Zasso
 | first2 = F.
 | last2 = Fossati
 | first3 = I.
 | last3 = Viola
 | title = Twisted flow wind tunnel design for yacht aerodynamic studies
 | series = 4th European and African Conference on Wind Engineering
 | year = 2005
 | pages = 350–351
 | place = Prague
 | url = http://www.research.ed.ac.uk/portal/files/17535106/Zasso_etal_EACWE4.pdf }}</ref>

Gusts may be predicted by the same value that serves as an exponent for wind shear, serving as a gust factor. So, one can expect gusts to be about 1.5 times stronger than the prevailing wind speed (a 10-knot wind might gust up to 15 knots). This, combined with changes in wind direction suggest the degree to which a sailing craft must adjust sail angle to wind gusts on a given course.<ref name=Hsu>
{{cite web
 | last = Hsu
 | first = S. A.
 | title = An Overwater Relationship Between the Gust Factor and the Exponent of Power-Law Wind Profile
 | work = Mariners Weather Log
 | volume = 52
 | issue = 1
 | publisher = National Oceanic and Atmospheric Administration
 | date = April 2008
 | url = http://www.vos.noaa.gov/MWL/apr_08/overwater.shtml
 | access-date = 19 March 2015 }}
</ref>