Editing Speed of sound (section)

==Altitude variation and implications for atmospheric acoustics==
[[File:Comparison US standard atmosphere 1962.svg|thumb|Density and pressure decrease smoothly with altitude, but temperature (red) does not. The speed of sound (blue) depends only on the complicated temperature variation at altitude and can be calculated from it since isolated density and pressure effects on the speed of sound cancel each other. The speed of sound increases with height in two regions of the stratosphere and thermosphere, due to heating effects in these regions.]]

In the [[Earth's atmosphere]], the chief factor affecting the speed of sound is the [[temperature]]. For a given ideal gas with constant heat capacity and composition, the speed of sound is dependent ''solely'' upon temperature; see ''{{section link|#Details}}'' below. In such an ideal case, the effects of decreased density and decreased pressure of altitude cancel each other out, save for the residual effect of temperature.

Since temperature (and thus the speed of sound) decreases with increasing altitude up to {{val|11|u=km}}, sound is [[refraction|refracted]] upward, away from listeners on the ground, creating an [[acoustic shadow]] at some distance from the source.<ref name="Everest2001">{{cite book
| last = Everest
| first = F.
| title = The Master Handbook of Acoustics
| publisher = McGraw-Hill
| location = New York
| year = 2001
| isbn = 978-0-07-136097-5
| pages = 262–263
}}</ref> The decrease of the speed of sound with height is referred to as a negative [[sound speed gradient]].

However, there are variations in this trend above {{val|11|u=km}}. In particular, in the [[stratosphere]] above about {{val|20|u=km}}, the speed of sound increases with height, due to an increase in temperature from heating within the [[ozone layer]]. This produces a positive speed of sound gradient in this region. Still another region of positive gradient occurs at very high altitudes, in the [[thermosphere]] above {{val|90|u=km}}.