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Template:Convert/Mach converts speeds of objects in Earth's atmosphere in Mach number with other units, where the optional parameter 3 (or 4) can be the altitude above (or below) mean sea level, expressed in feet. Some examples are shown below.

Usage: {{convert|1|Mach}} → Mach 1 (761 mph; 1,225 km/h)
  {{convert|2.0|Mach}} → Mach 2.0 (1,522.4 mph; 2,450.1 km/h)
  {{convert|3.3|Mach}} → Mach 3.3 (2,512.0 mph; 4,042.6 km/h)
  {{convert|3.3|Mach|90,000}} → Mach 3.3 (2,214.6 mph; 3,564.1 km/h)
  {{convert|3.3|Mach|90,000|sigfig=6}} → Mach 3.3 (2,214.63 mph; 3,564.10 km/h)
  {{convert|810|mph|Mach}} → 810 miles per hour (Mach 1.1 )
  {{convert|810|mph|Mach|sigfig=4}} → 810 miles per hour (Mach 1.064 )

The conversion depends on the altitude, passed as parameter 3 or 4 measured in feet (default: 0), and the temperature (ignored), where the temperature at higher altitudes is likely to be fairly constant. Hence, only the altitude is an optional parameter, when converting from Mach number into other speed units. The altitude cannot be specified in reverse conversions, to Mach number, from other units, and the altitude is only accepted as feet, not meters.

Note that the speed of sound alternately decreases then increases, and again decreases then increases, at successively higher altitudes in the Earth's atmosphere. From sea level, in the troposphere, the speed decreases with rising altitude until the tropopause, at 40,000 feet (8 mi; 12 km), where Mach 1 is 660.1 miles per hour (1,062.3 km/h), for most commercial jet airliners. However, the speed increases (as a reverse change) in the stratosphere, reaching 737.7 miles per hour (1,187.2 km/h) at altitude 160,000 feet (30 mi; 49 km). Then, the speed re-reverses to again decrease in the mesosphere, down to 613.2 mph (986.8 km/h) at altitude 285,000 feet (54 mi; 87 km). However, in the thermosphere, the speed of sound re-re-reverses to increase (as in the stratosphere), and reaches 688.9 mph (1,108.7 km/h) at altitude 400,000 feet (76 mi; 122 km). The highest speeds are below sea level, as in low-lying canyon areas, and the template allows a lowest altitude of -15,000 (feet) in parameter 3 or 4.