Definition
An altimeter error condition in which an aircraft flies from an area of higher atmospheric pressure into an area of lower atmospheric pressure without resetting the altimeter. Because the altimeter senses the lower pressure as a higher altitude, the instrument over-reads, and the aircraft is actually lower than indicated.
Plain English
If you fly into air where the pressure is lower than where you started, and you don't update your altimeter, the altimeter will tell you you're higher than you really are. You're actually flying lower than the number on the dial.
Context Anchor
Seen in basic aerodynamics when learning how a wing produces lift.
Why Pilots Care
This pressure difference is fundamental to understanding lift and safe flight operations.
Grounding Statement
Picture the wing moving through the air with less push on top and more push underneath; the uneven push helps raise the wing.
Intuition Check
Low Pressure Above does not mean there is no air above the wing, and it does not mean the wing is lifted by suction alone. It means the air above the wing is pressing less hard than the air below it.
Example Sentence 1
Heading west into the approaching low-pressure system, the instructor reminded the student about low pressure above and had her request a current altimeter setting before continuing.
Example Sentence 2
At higher angles of attack, the low pressure above intensifies until the critical angle is reached.