Plain English Meaning

Hot air is thinner and cold air is thicker. The hotter it gets, the fewer air molecules occupy the same space, and that thinner air gives the aircraft less to work with.

Full Definition

The principle that air density changes inversely with temperature when pressure is held constant: as air temperature rises, the air molecules spread out and density decreases; as air temperature falls, the molecules pack closer together and density increases. Higher temperatures therefore reduce aircraft performance because the wing, propeller, and engine all rely on dense air.

Context Anchor

Seen in aircraft performance discussions, especially when comparing takeoff, , and on hot versus cool days.