Supersonics is an important branch of aerodynamics that concerns phenomena that arise when the velocity of a solid body exceeds the speed of sound in the medium, usually air, in which it is traveling. The speed of sound in the atmosphere varies with humidity, temperature, and pressure. Because the speed of sound, being thus variable, is a critical factor in aerodynamic equations, it is represented by a so-called Mach number, named after the Austrian physicist and philosopher Ernst Mach, who pioneered the study of ballistics. The Mach number is the speed of the projectile or aircraft 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 1220 km/h (about 760 mph) represents a Mach number of one, that is, M-1. The same speed in the stratosphere, because of differences in density, pressure, and temperature, would correspond to a Mach number of M-1.16. By designating speeds by Mach number, rather than by kilometers or miles per hour, a more accurate representation of the actual conditions encountered in flight can be obtained and thus making supersonics an important subject in space exploration.