The speed of sound, about 761 miles per hour at sea level in dry air at 59 degrees Fahrenheit, varied depending on temperature, altitude, and humidity. It was long thought to be a physical limit on the maximum speed of an object moving through the air. As an airplane flying at sea level in dry air approached Mach 1, or 100 percent of the local speed of sound, air molecules in front of the flying plane piled up and compressed, forming a shock wave, the same phenomenon that caused the noise associated with the crack of a bull whip or the firing of a bullet.
