Which factor would tend to increase the density altitude at a given airport?

• A. An increase in ambient pressure
• B. An increase in ambient temperature
• C. A decrease in humidity
• D. A decrease in altitude

Answer: (A)

Density altitude is a way of expressing air density as an altitude in the standard atmosphere. When the air is warmer, it becomes less dense, so the density altitude increases. That thinner air means less oxygen molecules and fewer air molecules for lift and propulsion to work with, which is why performance suffers on hot days: engine power, takeoff distance, and climb rate all decrease as density altitude rises.

Looking at the other factors: higher ambient pressure makes the air denser, which lowers density altitude; lower altitude also makes the air denser, lowering density altitude; and increasing humidity actually lowers air density (moist air is lighter than dry air), which would raise density altitude, while decreasing humidity would do the opposite. So the factor that tends to increase density altitude is a rise in ambient temperature.