With respect to temperature differences in air mass, what will happen at higher altitudes?

At higher altitudes, a significant phenomenon occurs: the local speed of sound decreases. This is primarily due to the decreasing temperature with altitude in the troposphere. The speed of sound is directly related to the temperature of the air; as air temperature decreases, the speed of sound in that air also decreases.

In warmer air, molecules move more quickly, allowing sound waves to travel faster. Conversely, at higher altitudes where the air is colder, the movement of air molecules slows down, resulting in a lower speed of sound. This property is crucial for pilots as it affects their flight dynamics and performance calculations.

The other options do not represent the effects observed at higher altitudes accurately. For instance, stall speed typically increases at higher elevations due to reduced air density, not a decrease. Engine efficiency can be adversely affected by thinner air, and while turbulence can occur at various altitudes, it's not a guaranteed consequence strictly tied to higher altitudes. Thus, the correct choice highlights the relationship between altitude, temperature, and the speed of sound effectively.