Speed of Sound

Speed of Sound is the speed at which sound waves propagate through air. At sea level in standard conditions (15°C), it is approximately 661 knots (1,225 km/h). The speed of sound decreases with temperature and altitude — at FL350 (-56.5°C), it is approximately 573 knots.

The speed of sound is the reference for Mach number: Mach 1.0 equals the local speed of sound. Commercial jets cruise at Mach 0.78-0.85, well below Mach 1.0 to avoid transonic drag rise and structural concerns.