Definition
A condition in which the density altitude is significantly higher than the airport's actual elevation, meaning the air is less dense than standard. It occurs when some combination of high field elevation, high temperature, high humidity, or low atmospheric pressure causes the air to behave — from the airplane's perspective — as if the airport were located at a much higher altitude than it really is.
Plain English
The air is thinner than it should be for the airport's elevation. The airplane reacts as though it were taking off from a much higher field, even though the runway is where it has always been.
Context Anchor
Encountered during takeoff and climb planning, especially at hot, high-elevation airports or any time the airplane feels slow to accelerate and climb.
Derivation
"Density" comes from Latin densus, meaning thick or packed closely together. "Altitude" comes from Latin altus, meaning high. Density altitude is therefore the altitude at which the surrounding air would normally be packed this thinly. "High" density altitude is the slightly counterintuitive label for thin air — the density altitude number is high even though the air itself is sparse.
Why Pilots Care
High-density altitude reduces engine power, propeller thrust, and wing lift, resulting in longer takeoff rolls, lower climb rates, and the need for greater obstacle clearance margins.
Grounding Statement
On a hot day at a high airport, the airplane may act like it is already much higher in the sky before it even leaves the runway.
Intuition Check
Do not read high-density altitude as “high-density air.” It means a high density-altitude value, which usually means the air is thinner, not thicker.
Example Sentence 1
On a 95°F afternoon at a 5,000-foot mountain airport, the pilot calculated a high density altitude and added several hundred feet to the expected takeoff roll.
Example Sentence 2
Even at a 1,500-foot airport, the high-density altitude forced the pilot to wait for cooler temperatures before attempting a short-field departure.