Definition
An altimeter error condition in which the actual atmospheric pressure at the aircraft's location is higher than the pressure setting in the altimeter, causing the altimeter to indicate a lower altitude than the aircraft is actually flying. This typically occurs when an aircraft flies from an area of lower pressure into an area of higher pressure without updating the altimeter setting.
Plain English
If the air pressure underneath the aircraft is higher than what the altimeter is set to, the altimeter shows the aircraft as being lower than it really is. The aircraft is actually higher than the instrument suggests.
Context Anchor
Seen in basic aerodynamics explanations of how a wing produces lift.
Derivation
Pressure comes from a Latin word meaning “to press.” In this term, it means air pressing against a surface. “Below” points to the lower side of the wing, where that pressing force is greater than it is above the wing.
Why Pilots Care
Indicates generally stable air with reduced turbulence and good visibility, but may bring haze or fog near the surface.
Grounding Statement
In normal flight, picture the air under the wing pressing upward more strongly than the air above the wing presses downward.
Intuition Check
Do not read this as a weather “high pressure” system below the airplane. Here it means higher pressure on the underside of the wing compared with the pressure above it.
Example Sentence 1
Flying into an area of high pressure below without resetting the altimeter, the pilot was actually 200 feet higher than the instrument indicated.
Example Sentence 2
With high pressure below the aircraft, the altimeter tended to read slightly higher than actual altitude.