Definition
The fixed rate at which a parcel of unsaturated (dry) air cools as it rises and warms as it descends, due solely to expansion or compression with changing pressure. The accepted value is approximately 3 °C (5.4 °F) per 1,000 feet.
Plain English
When a pocket of dry air rises, it cools at a steady, predictable rate of about 3 °C for every 1,000 feet it climbs — and warms at the same rate as it sinks. No heat is added or removed; the temperature change comes only from the air expanding or being squeezed.
Context Anchor
Seen in weather theory and atmospheric stability discussions, especially when comparing rising air with the air around it.
Derivation
‘Adiabatic’ comes from the Greek adiabatos, meaning ‘not passable’ — referring to heat not passing in or out. ‘Lapse’ means a gradual decrease. So the phrase literally describes how dry air cools off as altitude increases, with no heat exchange involved.
Why Pilots Care
It is used to determine atmospheric stability, which affects turbulence, cloud formation, and the likelihood of thunderstorms along a route.
Analogy
A bicycle pump can get warm when it squeezes air. Sinking air is also squeezed by higher pressure and warms; rising air expands in lower pressure and cools.
Grounding Statement
Imagine a balloon of dry air pushed upward — as outside pressure drops, the balloon expands, and that expansion alone cools the air inside by about 3 °C per 1,000 feet.
Intuition Check
Dry does not mean the air has no moisture at all; it means the air is not saturated. Adiabatic does not mean the sun or ground is heating the air; the temperature changes because the air expands or is squeezed. Lapse rate is about temperature change with height, not time passing.
Example Sentence 1
Because the air mass was unsaturated, the briefer used the dry adiabatic lapse rate to estimate how much it would cool as it lifted over the mountains.
Example Sentence 2
Knowing the dry adiabatic lapse rate helped the student predict that scattered cumulus clouds would form by midday.