Moist and Dry Adiabatic Processes

How unsaturated and saturated air parcels change temperature as they ascend or descend without exchanging heat with their surroundings.

Definition

An adiabatic process is a change in an air parcel's state with no heat exchange with the environment; it is dry when the air is unsaturated and moist when condensation releases latent heat during ascent.

Scope

Covers the dry adiabatic lapse rate, the saturated or moist adiabatic lapse rate, potential temperature and equivalent potential temperature as conserved quantities, the lifting condensation level, and the role of latent heat release in modifying parcel cooling.

Core questions

• Why does the dry adiabatic lapse rate have a fixed value of about 9.8 degrees Celsius per kilometre?
• How does latent heat release reduce the cooling rate of saturated ascent?
• What thermodynamic quantities are conserved during adiabatic motion?

Key theories

Adiabatic lapse rates

Applying the first law to an adiabatically rising parcel yields a constant dry lapse rate, and adding latent heat release on saturation yields a smaller, temperature-dependent moist lapse rate.

Mechanisms

As an unsaturated parcel rises it expands and cools at the dry adiabatic rate set by gravity and the specific heat of air. Potential temperature, the temperature a parcel would have if brought adiabatically to a reference pressure, is conserved in dry ascent. Once the parcel reaches saturation at the lifting condensation level, condensation releases latent heat that partly offsets expansion cooling, so the moist adiabatic rate is smaller and varies with temperature; equivalent potential temperature is conserved through this moist process.

Clinical relevance

Adiabatic concepts are used to estimate cloud base height, predict convective cooling and warming, and trace air masses on thermodynamic diagrams.

History

The distinction between dry and pseudo-adiabatic moist processes and the conserved quantity equivalent potential temperature were developed by von Bezold and Normand around the turn of the twentieth century and remain foundational to convective meteorology.

Key figures

• Wilhelm von Bezold
• Craig Bohren

Seminal works

• bohren1998
• iribarne1981

Frequently asked questions

Why is the moist adiabatic lapse rate smaller than the dry rate?

When saturated air rises, water vapour condenses and releases latent heat, partly offsetting the cooling from expansion, so saturated air cools more slowly with height than dry air.