What is the speed of sound at sea level in mph and in knots? Also what is the speed of sound at the
Tropopause in mph and knots?
- question from Brittney
To account for this behavior, aerospace engineers make use of what is called the standard atmosphere. This standard atmosphere is based on scientific atmospheric data collected at different locations within the atmosphere. This data was then used to create a series of equations that mathematically model the values of key atmospheric properties, such as temperature, density, and speed of sound. The results of this model provide engineers with "average" atmospheric properties on a so-called standard day.
The standard atmospheric model tells us that the speed of sound, or Mach 1, at sea level is:
But now we face another problem, because aircraft do not typically spend much time flying at sea level. They instead cruise tens of thousands of feet above the Earth's surface where the speed of sound changes. This change in speed of sound is directly related to the change in temperature as altitude increases. This temperature change can be observed below.
Furthermore, the temperature (T) and speed of sound (a) are directly related by the following equation:
Having established that temperature changes with altitude and speed of sound is directly proportional to temperature, it is now clear that the speed of sound changes as altitude increases. As illustrated above, the temperature decreases at a linear rate up to about 11 km (6.8 mi) where the Tropopause begins. This region of the atmosphere is marked by constant temperature, and therefore constant speed of sound. The Tropopause extends up to about 20 km (12.4 mi), so the speed of sound does not change throughout this entire 9 km (5.6 mi) thick region. Scientific measurements and the standard atmospheric model have established that the speed of sound, or Mach 1, within this realm is:
I was on a flight ... when the TV screen showed a speed of well over 700 mph, and the pilot told us that we were being pushed by one of the fastest tail winds he had ever experienced. ... Did I fly faster than the speed of sound?
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