Which phenomenon is affected by the decrease in air density at higher altitudes?

The correct answer is that both heat transfer and sound propagation are affected by the decrease in air density at higher altitudes.

As altitude increases, the density of air decreases, which can significantly influence how both heat and sound behave.

For heat transfer, the decrease in air density at higher altitudes leads to lower thermal conductivity, which can alter how effectively heat is transferred between surfaces and the surrounding environment. This is particularly significant in processes such as convection, where warmer air rises and cooler air descends. The thinning of air reduces the mass of air that can carry heat, hence affecting the overall heat transfer dynamics.

Regarding sound propagation, sound travels through air as a wave, and its speed is influenced by the medium it travels through. In less dense air, like that found at higher altitudes, sound waves can travel more slowly because the molecules are further apart, resulting in less efficient transmission of the sound waves. Additionally, higher altitudes can also affect the frequency and intensity of sounds, making them less perceivable to the human ear.

Thus, both phenomena—heat transfer and sound propagation—are intricately linked to the density of air at varying altitudes, affirming that the answer encompasses both aspects.