Definition
Pressure altitude corrected for non-standard temperature. It is the altitude at which the standard atmosphere would have the same air density as the air the aircraft is currently flying through. As temperature rises, humidity rises, or pressure falls, density altitude increases and the air becomes thinner.
Plain English
How thin or thick the air actually feels to the airplane right now, expressed as an altitude. On a hot day at a low airport, the air can feel as thin as the air thousands of feet higher up, and the airplane performs as if it were up there.
Context Anchor
You will see density altitude in takeoff, climb, landing, and go-around performance discussions, especially when operating from high-elevation airports or on hot days.
Derivation
Density refers to how tightly packed the air molecules are. Altitude here is borrowed as a yardstick: instead of describing thin air in technical units, we describe it as the altitude where the standard atmosphere would feel the same.
Why Pilots Care
Higher density altitude reduces engine power output, propeller thrust, and wing lift, resulting in longer takeoff rolls, reduced climb performance, and the need for greater runway length or obstacle clearance margins.
Analogy
Think of the airplane as trying to push against the air. In thin air, there is less to push against, so the airplane has a harder time speeding up, lifting off, and climbing.
Grounding Statement
On a 95 degree day at a 5,000 foot airport, the airplane may perform as if it were taking off from 8,000 feet -- even though the runway elevation has not changed.
Intuition Check
Density altitude is not the airport's actual height above sea level. It is a performance altitude based on how thin the air is.
Example Sentence 1
The pilot calculated a density altitude of 9,500 feet before departing the mountain strip on a hot summer afternoon.
Example Sentence 2
The instructor reminded the student that even at sea level a 35-degree-C day can produce a density altitude high enough to prevent a normal climb after liftoff.