Under what condition is pressure altitude and density altitude the same value?

• A. At standard temperature.
• B. At high humidity.
• C. In very cold air.
• D. At very low pressure.

Answer: (A)

Focus on how density and pressure relate to altitude. Pressure altitude is the altitude corresponding to a given pressure in the standard atmosphere. Density altitude is the altitude in the standard atmosphere at which the air density matches the current air density. They become the same when the air temperature is exactly what the ISA (standard atmosphere) specifies for that height.

At standard temperature, the air’s density follows the standard model for that pressure, so the current density matches the standard density at that height. That makes the altitude that corresponds to the current density equal to the altitude corresponding to the standard pressure, i.e., pressure altitude equals density altitude.

When the temperature is not standard, density deviates from the standard value, so the densities don’t line up and the two altitudes differ. Humidity or very low/high pressure also shift density away from the standard expectation, which is why those conditions don’t yield equality.