Atmospheric Thermodynamics

Treating a bubble of air as a thermodynamic system explains why mountains are cold, why air sinking off a range warms, and why the release of latent heat can turn a rising parcel into a towering storm.

Definition

Atmospheric thermodynamics is the study of the energy transformations of air parcels, particularly the adiabatic expansion and compression that govern their temperature and the latent-heat exchanges accompanying changes in the phase of water.

Scope

This topic covers the application of the first law of thermodynamics to atmospheric air parcels, the dry and moist adiabatic lapse rates, conserved variables such as potential and equivalent potential temperature, and the thermodynamic diagrams used to analyze soundings.

Core questions

• How does the first law of thermodynamics describe a rising or sinking air parcel?
• What are the dry and moist adiabatic lapse rates and why do they differ?
• Why are potential and equivalent potential temperature useful conserved quantities?
• How do thermodynamic diagrams represent the state and processes of the atmosphere?

Key theories

Adiabatic lapse rates

An unsaturated parcel cools on ascent at the constant dry adiabatic rate, while a saturated parcel cools more slowly at the moist adiabatic rate because condensation releases latent heat into the parcel.

Conserved thermodynamic variables

Potential temperature is conserved in dry adiabatic motion and equivalent potential temperature in moist adiabatic motion, so these quantities label air parcels and reveal their origins and stability.

Mechanisms

Because air is a poor conductor and parcels move quickly, vertical motion is well approximated as adiabatic: a rising parcel expands and cools, a sinking one compresses and warms. The first law sets the rate of cooling, the dry adiabatic lapse rate, until saturation, after which latent heat from condensation reduces it to the moist adiabatic rate. Potential temperature, which removes the effect of pressure, is conserved in dry motion, and equivalent potential temperature in moist motion, providing tracers that are read directly from thermodynamic diagrams such as the tephigram or skew-T.

Clinical relevance

Atmospheric thermodynamics underlies the interpretation of soundings to assess stability and forecast convection, the prediction of foehn and chinook warming downslope of mountains, and the calculation of cloud bases and convective energy used daily in operational forecasting.

History

The application of classical thermodynamics to the atmosphere developed in the late nineteenth and early twentieth centuries, drawing on the work of Helmholtz and others, and included the introduction of potential temperature and the design of thermodynamic diagrams such as the tephigram by Napier Shaw and the later skew-T log-p diagram, which remain standard tools for analyzing the vertical structure of the atmosphere.

Key figures

• William Napier Shaw
• Hermann von Helmholtz
• Vilhelm Bjerknes

Seminal works

• bohren1998
• iribarne1981

Frequently asked questions

Why does air cool faster when it is dry than when it is forming clouds?

Dry air cools at the dry adiabatic lapse rate as it rises, but once a parcel saturates and clouds form, condensation releases latent heat that partly offsets the cooling, so the parcel cools more slowly at the moist adiabatic rate.

What is potential temperature?

Potential temperature is the temperature an air parcel would have if brought adiabatically to a standard pressure; because it stays constant during dry vertical motion, it is a convenient label that identifies and tracks parcels of air.