The term "supersonic" refers primarily to the flight or motion of any solid body through a gas at a velocity that exceeds the velocity of sound in that gas. When we speak of sound in this connection, we have in mind the pressure variations that move outward in all directions, in the form of waves, from a "disturbance" such as the tooting of a horn or the roar of a lion. The waves in question are usually very weak. When they strike our eardrums, they set up vibrations that are recorded in the brain so that we "hear" the disturbance that set off the waves. Of course, these pressure variations, or sound waves, as they are generally called, exist whether or not our eardrums and result in the sensation of hearing.
Sound waves traveling through the atmosphere near the Earth's surface have a velocity that decreases continually with altitude. It ranges from 1,120 feet per (762 miles per hour) at sea level. standard conditions of temperature and pressure, to 970 feet per second (660 per hour) at an altitude of 36,000. Then it remains constant to 82,000 feet altitude, after which it starts to increase.
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