Sound is a pressure wave of small magnitude and its speed of propagation in the fluid is called the speed of sound. The airflow around a body creates higher air pressures in the vicinity which travel upstream, giving advance warning of the presence of the body.
Because of this pressure wave the air moves in a curved path ahead of the body, passing around it with the minimum of disturbance.
If the airspeed is greater than the speed of sound, these warning signals cannot propagate upstream at all and no warning is given – this is called supersonic flow. In this situation the air must change direction suddenly when it encounters the body. If the deviation asked of it is small it does so, producing a small-amplitude shock wave attached to the body. If the deviation is large, a large-amplitude shock wave can move ahead of the body (large amplitude waves travel faster) and behind this the air is slowed to subsonic speed. These shock waves are commonly called sonic booms.
With such different flow systems on either side of the speed of sound, it becomes imperative to know whether the airspeed is above or below this value. Unfortunately, the speed of sound does not have a unique value, but varies with temperature. In this situation it becomes convenient to divide the airspeed by the speed of sound – this ratio is named the Mach number for Ernest Mach, an Austrian scientist who studied the flight of bullets.
When airspeeds increase still further, the rise in temperature and pressure behind shock waves becomes so large that the air dissociates, that is, some of the molecules of nitrogen and oxygen which constitute air break down into atoms. Behind the body, temperatures and pressures decrease and the molecules re-form. This is hypersonic flow.
Atmospheric pressure at sea level is caused by the weight of air above the point of measurement. Consequently, at higher altitudes pressure and density decrease. There comes a height – over 50 miles (80 km) above sea level – where the density is so low that the mean free path (the average distance between collisions of the molecules) is the same order of magnitude as the body under consideration. Air no longer behaves as an entity (usually called a continuum) and pressure and forces become the result of individual molecular collisions with the body surface. This part of the subject is called free molecular or Newtonian flow. There is no sharp division between continuum and molecular flow, rather a progressive change, and this part of the subject is called slip flow.