Definition
A region where the local air pressure is lower than the surrounding ambient (free-stream) pressure. On an airfoil, negative pressure develops above the wing as air accelerates over the curved upper surface, and this pressure difference between the upper and lower surfaces is the primary source of lift.
Plain English
An area where the air pressure is less than the air pressure around it. Above a wing in flight, the air moving over the top is at lower pressure than the air below, and that difference is what holds the airplane up.
Context Anchor
Seen in airfoil and lift discussions, especially when describing the lower pressure that forms over the upper surface of a wing.
Derivation
In everyday speech, 'negative' often means 'bad' or 'less than zero.' Here it simply means 'lower than the reference value' — in this case, lower than the surrounding atmospheric pressure. The air is not at a vacuum; it is just at a reduced pressure relative to the air nearby.
Why Pilots Care
Negative pressure on the top of a wing pulls the wing upward and is a major contributor to total lift.
Grounding Statement
Picture air moving over the top of a wing and pressing less hard there than the air below, helping create lift.
Intuition Check
Negative pressure does not mean pressure below zero or a total vacuum. Here it means pressure below the reference pressure being compared.
Example Sentence 1
As the wing's angle of attack increases, the negative pressure on the upper surface grows stronger, producing more lift.
Example Sentence 2
During preflight, the student pilot learned that negative pressure on the upper surface is what allows the wing to generate most of its lift.