Supersonic phenomena

   Supersonics are a branch of physics dealing with the phenomena arising when a solid body exceeds the speed of sound in the medium in which it is traveling; usually the medium is air. The speed of sound in air is dependent upon several factors, including the temperature, humidity, density, and altitude. Because the speed of sound, being thus vari¬able, is a critical factor in aerodynamic equations, it is represented by a Mach number. The Mach number is the speed of the projectile or plane with reference to the ambient atmosphere, divided by the speed of sound in the same medium and under the same conditions. Thus, at sea level, under standard
conditions of humidity and temperature, a speed of about 760 miles per hour represents a Mach number of one, that is, M=l. The same speed in the stratosphere, because of difference in density and temperature, would correspond to a Mach number of M=1.16. By designating speeds by Mach number, rather than by feet per second, or miles per hour, it is possible to obtain a more accurate representation of the actual conditions encountered in flight.