Definition
The rate at which atmospheric pressure decreases with an increase in altitude. In the lower atmosphere, pressure drops by approximately 1 inch of mercury (1 inHg) for every 1,000 feet of altitude gained.
Plain English
As you climb, the air gets thinner and pressure falls. The pressure lapse rate is how fast that pressure drops with height — roughly 1 inch of mercury for every 1,000 feet near the surface.
Context Anchor
Seen in altimeter and pitot-static instrument discussions, especially when explaining how an altimeter turns outside air pressure into an altitude indication.
Derivation
‘Lapse’ comes from the Latin lapsus, meaning ‘a slip’ or ‘fall.’ In atmospheric terms, it describes how a quantity falls off as altitude increases. So a pressure lapse rate is simply how fast pressure ‘falls away’ as you go up.
Why Pilots Care
Using the wrong lapse rate produces altimeter errors that can lead to incorrect altitude awareness.
Grounding Statement
Climb 1,000 feet near sea level, and the air pressure around you drops by about 1 inch of mercury — that drop is the pressure lapse rate at work.
Intuition Check
Do not read “lapse” as a mistake or a failure here. In this term, it means a decrease: pressure falls as altitude increases.
Example Sentence 1
Because of the standard pressure lapse rate, the altimeter reads about 1,000 feet higher for each 1 inHg drop in pressure.
Example Sentence 2
Knowing the actual pressure lapse rate helped determine true altitude during the approach.