Editing Mach number (section)

== Overview ==
[[File:Comparison US standard atmosphere 1962.svg|thumb|The speed of sound (blue) depends only on the temperature variation at altitude (red) 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.]]
Mach number is a measure of the [[compressible flow|compressibility characteristics of fluid flow]]: the fluid (air) behaves under the influence of compressibility in a similar manner at a given Mach number, regardless of other variables.<ref name=NASA>{{cite web |url=http://www.grc.nasa.gov/WWW/k-12/airplane/mach.html |work=[[NASA]] |title=Mach Number |editor=Nancy Hall}}</ref> As modeled in the [[International Standard Atmosphere]], dry air at [[mean sea level]], standard temperature of {{convert|15|C|F}}, the speed of sound is {{convert|340.3|m/s|ft/s mph km/h kn|sp=us|sigfig=5}}.<ref>Clancy, L.J. (1975), Aerodynamics, Table 1, Pitman Publishing London, {{ISBN|0-273-01120-0}}</ref> The speed of sound is not a constant; in a gas, it increases proportionally to the square root of the [[absolute temperature]], and since atmospheric temperature generally decreases with increasing altitude between sea level and {{convert|11000|m|ft|sp=us|sigfig=5}}, the speed of sound also decreases. For example, the standard atmosphere model lapses temperature to {{convert|-56.5|C|F}} at {{convert|11000|m|ft|sp=us|sigfig=5}} altitude, with a corresponding speed of sound (Mach{{nbsp}}1) of {{convert|295.0|m/s|ft/s mph km/h kn|sp=us|sigfig=4}}, 86.7% of the sea level value.