As altitude increases, atmospheric pressure changes how, and what is the effect on gas density?

• A. Pressure increases with altitude; density increases.
• B. Pressure stays the same; density unchanged.
• C. Pressure decreases with altitude; density increases.
• D. Pressure decreases with altitude; density decreases.

Answer: (D)

As you go higher, the air becomes more sparse because there are fewer air molecules pushing down on you, so atmospheric pressure decreases with altitude. The density of the gas is how much mass sits in a given volume, and it’s tied to pressure through the gas relationships: at a given temperature, lower pressure means the gas expands and becomes less dense (ρ = P/(R_specific T)). So when altitude increases, both pressure and density tend to decrease. In the real atmosphere temperature changes with height, but the overall trend still has lower density at higher altitudes.