Definition
The rate at which atmospheric pressure decreases with altitude in the standard atmosphere, defined as approximately 1 inch of mercury (1 inHg) per 1,000 feet of altitude gain in the lower atmosphere. This rate is one of the fixed assumptions built into the International Standard Atmosphere (ISA) and is the basis on which pressure altimeters are calibrated.
Plain English
It's the assumed, agreed-upon rate at which air pressure drops as you climb. In the standard atmosphere, every 1,000 feet you go up, the pressure falls by about 1 inch of mercury. Altimeters are built around this assumption.
Context Anchor
You see this in sensitive altimeter discussions, where pressure changes are converted into altitude indications.
Derivation
Lapse' comes from the Latin lapsus, meaning a slip or fall. Here it describes how pressure 'falls off' as altitude increases. 'Standard' refers to the agreed reference atmosphere used worldwide for calibration, not to actual conditions on any given day.
Why Pilots Care
Pilots use this rate to understand how altimeters translate pressure into altitude and to set correct reference pressures for accurate readings.
Analogy
Think of it like a simple ruler for pressure: near sea level, about 1 inHg of pressure change lines up with about 1,000 feet of altitude change in the standard model.
Grounding Statement
When an airplane climbs, there is less air above it, so the surrounding air pressure drops and the altimeter senses that drop.
Intuition Check
Standard does not mean the actual weather pressure outside today. Lapse does not mean a delay here; it means the expected drop in air pressure as altitude increases.
Example Sentence 1
Because altimeters are calibrated to the standard pressure lapse rate, a 1 inHg change in the Kollsman setting shifts the indicated altitude by about 1,000 feet.
Example Sentence 2
Knowing the standard pressure lapse rate helps verify whether an altimeter setting is reasonable for the current field elevation.