Definition
The rate at which a parcel of saturated (cloud-forming) air cools as it rises, or warms as it descends, when no heat is exchanged with the surrounding air. Because condensation releases latent heat into the rising parcel, the cooling rate is slower than for dry air — typically around 2 to 3 °C per 1,000 feet, varying with temperature and moisture content.
Plain English
When air that is already saturated rises, it cools more slowly than dry air does, because the moisture condensing into cloud droplets gives off heat that partly offsets the cooling.
Context Anchor
Seen in weather and atmospheric stability discussions, especially when comparing rising moist air with surrounding air to understand clouds, turbulence, and storm growth.
Derivation
‘Moist’ refers to air that is saturated with water vapor. ‘Adiabatic’ comes from the Greek adiabatos, meaning ‘not passable’ — describing a process where no heat passes in or out of the parcel. ‘Lapse rate’ means the rate at which temperature decreases with altitude. Together: the cooling-with-altitude rate of saturated air rising on its own, with no outside heat exchange.
Why Pilots Care
Determines whether rising air will form tall clouds, turbulence, or icing layers that affect flight safety and route planning.
Analogy
Like a damp sponge being lifted: squeezing out water releases warmth, so the sponge cools more slowly than a dry one.
Grounding Statement
Picture a rising parcel of cloud: as water vapor inside it condenses into droplets, that condensation gives off a little warmth, so the parcel cools more gently than dry air would on the way up.
Intuition Check
Moist does not simply mean “rainy” here; it means the air is saturated enough that condensation affects its temperature change. Lapse does not mean a mistake; here it means a temperature change with altitude.
Example Sentence 1
Once the rising air reached saturation, it began cooling at the moist adiabatic lapse rate, which let the cumulus build much higher than the surrounding dry air would have allowed.
Example Sentence 2
Knowing the moist adiabatic lapse rate helped the pilot anticipate possible icing inside the cloud layer.