Airplanes move through a fluid. The density of that fluid—air—changes as the weather and altitude change. The density of the air that flows over the wings, past the propeller, and into the engine for combustion affects how the airplane performs. As altitude and temperature increase, the air’s density decreases (more humid air is also less dense, but performance charts rarely take this into account). Because the airplane senses only ambient air, and not a true altitude or temperature, it’s density altitude we rely on to tell us how the airplane will perform.
