The speed of sound fundamentally changes with altitude. This change is governed by the physical properties of the air column above Earth’s surface. Understanding how altitude affects the speed of sound requires looking at what makes sound travel and how the atmosphere’s temperature structure is layered. This variation is particularly important for high-speed flight.
What Controls the Speed of Sound
Sound travels as a wave of vibration transmitted through a medium, such as air, via the collision of molecules. The speed at which this transmission occurs is governed almost entirely by the temperature of the gas, which is a measure of the kinetic energy of its molecules. Warmer air means molecules are moving faster and more energetically, allowing them to transfer the sound energy to their neighbors more quickly. The speed of sound is directly proportional to the square root of the absolute temperature of the air.
A common misconception is that the drop in air pressure and density at high altitudes slows sound down. While pressure and density decrease significantly with height, these two effects nearly cancel each other out in an ideal gas like air. If the temperature remains constant, a decrease in pressure does not change the speed of sound because the density decreases proportionally. Therefore, temperature remains the dominant factor controlling the speed of sound in the atmosphere.
How Temperature Changes with Altitude
The temperature profile of the atmosphere is not uniform, leading to complex changes in the speed of sound as altitude increases. The lowest layer, the troposphere, extends from the surface up to about 11 kilometers (36,000 feet) and is where nearly all weather occurs. Within the troposphere, the temperature generally decreases with increasing height.
This decrease follows the standard lapse rate, which is approximately 6.5 degrees C for every 1,000 meters (or 3.57 degrees F per 1,000 feet) of ascent. This consistent temperature drop is the primary reason the speed of sound decreases throughout the lower atmosphere. At the upper boundary of the troposphere, known as the tropopause, the temperature stabilizes at a minimum.
Above the tropopause lies the stratosphere, where the temperature profile reverses. The temperature remains relatively constant for a few kilometers before beginning to increase with height due to the absorption of ultraviolet radiation by the ozone layer. This temperature increase in the stratosphere means the speed of sound begins to stabilize and then rise again in the upper atmosphere.
Modeling Sound Speed Across Atmospheric Layers
By combining the physical relationship between temperature and sound speed with the atmosphere’s temperature profile, a model emerges for how the speed of sound changes with altitude. The steady temperature drop in the troposphere directly translates to a steady reduction in the speed of sound. On a standard day, the speed of sound is approximately 340 meters per second (about 761 mph) at sea level, where the temperature is 15 degrees C.
As an aircraft climbs through the troposphere, the sound speed decreases significantly, reaching its lowest point at the tropopause. This represents a reduction of nearly 15% from the sea-level value.
Beyond this altitude, in the lower stratosphere, the constant or rising temperature means the speed of sound either plateaus or begins a gradual increase. This demonstrates that the speed of sound is a dynamic value tied to the fluctuating thermal structure of the atmosphere.
Why This Matters for Aviation
The variation of the speed of sound with altitude is a fundamental consideration for high-speed aviation, particularly when discussing the Mach number. The Mach number is defined as the ratio of an aircraft’s true airspeed to the local speed of sound. Since the speed of sound decreases with altitude up to the tropopause, an aircraft flying at a constant true airspeed will experience an increasing Mach number as it climbs.
This effect is crucial for flight planning because every aircraft has a maximum operating Mach number that must not be exceeded to prevent structural damage from shockwaves. Pilots operating in the upper atmosphere must transition from managing their speed by indicated airspeed to managing it by Mach number to ensure safety and performance.