Editing Speed of sound (section)

===Effects due to wind shear===
The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is [[refraction|refracted]] upward, away from listeners on the ground, creating an [[acoustic shadow]] at some distance from the source.<ref name="Everest2001" /> Wind shear of 4 m/(s · km) can produce refraction equal to a typical temperature [[lapse rate]] of {{val|7.5|u=°C/km}}.<ref>{{cite book
| last = Uman
| first = Martin
| title = Lightning
| publisher = Dover Publications
| location = New York
| year = 1984
| isbn = 978-0-486-64575-9
| url-access = registration
| url = https://archive.org/details/trent_0116300718198
}}</ref> Higher values of wind gradient will refract sound downward toward the surface in the downwind direction,<ref>{{cite book
| last = Volland
| first = Hans
| title = Handbook of Atmospheric Electrodynamics
| publisher = CRC Press
| location = Boca Raton
| year = 1995
| isbn = 978-0-8493-8647-3
| page = 22
}}</ref> eliminating the acoustic shadow on the downwind side. This will increase the audibility of sounds downwind. This downwind refraction effect occurs because there is a wind gradient; the fact that sound is carried along by the wind is not important.<ref>{{cite book
| last = Singal
| first = S.
| title = Noise Pollution and Control Strategy
| publisher = Alpha Science International
| location = Oxford
| year = 2005
| isbn = 978-1-84265-237-4
| page = 7
| quote = It may be seen that refraction effects occur only because there is a wind gradient and it is not due to the result of sound being convected along by the wind.
}}</ref>

For sound propagation, the exponential variation of wind speed with height can be defined as follows:<ref>{{cite book
| last = Bies
| first = David
| title = Engineering Noise Control, Theory and Practice
| publisher = CRC Press
| location = London
| year = 2009
| isbn = 978-0-415-26713-7
| page = 249 |
quote = As wind speed generally increases with altitude, wind blowing towards the listener from the source will refract sound waves downwards, resulting in increased noise levels.
}}</ref>
<math display="block">\begin{align}
U(h) &= U(0) h^\zeta, \\
\frac{\mathrm{d}U}{\mathrm{d}H}(h) &= \zeta \frac{U(h)}{h},
\end{align}</math>
where
* ''U''(''h'') is the speed of the wind at height ''h'';
* ''ζ'' is the exponential coefficient based on ground surface roughness, typically between 0.08 and 0.52;
* ''dU''/''dH''(''h'') is the expected wind gradient at height ''h''.

In the 1862 [[American Civil War]] [[Battle of Iuka]], an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle,<ref>{{cite book
| last = Cornwall
| first = Sir
| title = Grant as Military Commander
| publisher = Barnes & Noble
| location = New York
| year = 1996
| isbn = 978-1-56619-913-1
| page = 92
}}</ref> because they could not hear the sounds of battle only {{val|10|u=km}} (six miles) downwind.<ref>{{cite book
| last = Cozens
| first = Peter
| title = The Darkest Days of the War: the Battles of Iuka and Corinth
| publisher = The University of North Carolina Press
| location = Chapel Hill
| year = 2006
| isbn = 978-0-8078-5783-0
}}</ref>