Definition
The speed at which a pressure disturbance (a sound wave) travels through a given medium. In air, it is not constant — it depends primarily on temperature. At standard sea-level conditions (15°C / 59°F), the speed of sound in air is approximately 661 knots (761 mph, or about 1,116 feet per second). It decreases as air temperature decreases, so it is lower at altitude.
Plain English
How fast a sound wave moves through the air. It changes with temperature — colder air slows it down, warmer air speeds it up.
Context Anchor
Seen in high-speed flight, propeller, turbine, and Mach number discussions, where aircraft or part speed is compared with how fast sound is moving through the surrounding air.
Why Pilots Care
It sets the reference for critical speeds where airflow changes behavior around the aircraft.
Grounding Statement
On a cold day, sound travels slower, so the same aircraft speed is closer to the speed of sound than it would be on a warm day.
Intuition Check
Do not treat the speed of sound as one fixed number. In aviation, it means the local speed of sound in the air around the aircraft, mainly set by temperature.
Example Sentence 1
Because the speed of sound decreases with altitude, a jet cruising at FL350 reaches its limiting Mach number at a lower true airspeed than it would near the ground.
Example Sentence 2
Lower temperatures at altitude reduced the speed of sound and narrowed the safe operating margin.