Definition
The speed at which sound travels through a given medium under specific conditions. In air, sonic speed varies with temperature: it is approximately 761 mph at sea level under standard conditions and decreases as air temperature drops with altitude.
Plain English
How fast sound moves through the air around the aircraft. It is not a fixed number — it changes with temperature, so it is slower at high altitude where the air is colder.
Context Anchor
Seen in high-speed aerodynamics, Mach number discussions, aircraft operating limits, and temperature effects at altitude.
Derivation
From the Latin sonus, meaning sound. Sonic speed is literally the speed of sound — the term is a direct label for that physical quantity.
Why Pilots Care
Reaching sonic speed introduces compressibility effects, shock waves, and a sharp rise in drag that affects handling and performance.
Grounding Statement
In colder air aloft, sound moves slower, so an aircraft can be closer to sonic speed than it would be at the same actual speed in warmer air.
Intuition Check
Sonic speed is not one fixed number for all flying. It means the local speed of sound in the air around the airplane, and that speed changes mainly with temperature.
Example Sentence 1
As the jet climbed into colder air, sonic speed dropped, so the same Mach number represented a lower true airspeed.
Example Sentence 2
At higher altitudes the lower temperature reduced sonic speed, so the plane reached Mach 1 at a slower indicated airspeed.