During a climb below the tropopause in ISA conditions, how do the Mach number and speed of sound behave?

In the context of climbing through the atmosphere below the tropopause under ISA (International Standard Atmosphere) conditions, the behavior of the Mach number and the speed of sound is influenced by the changes in temperature with altitude.

As an aircraft climbs in the lower to mid-troposphere, the temperature generally decreases. The speed of sound, which is dependent on the square root of the temperature in Kelvin, also decreases when the temperature drops. Thus, as an aircraft ascends and the temperature falls, the speed of sound decreases.

On the other hand, the Mach number, defined as the ratio of the true airspeed of the aircraft to the speed of sound, is affected by the decrease in speed of sound. If the aircraft maintains a constant true airspeed while climbing, the decrease in speed of sound results in an increase in the Mach number.

In summary, during a climb below the tropopause in ISA conditions, while the speed of sound decreases due to lower temperatures, the Mach number increases because the aircraft's true airspeed remains constant relative to the decreasing speed of sound.