Atmospheric Thermodynamics
The application of thermodynamic laws to air, governing temperature, pressure, moisture and the energy of vertical motions.
Definition
Atmospheric thermodynamics is the branch of atmospheric physics that applies the laws of thermodynamics to air and water vapour to describe their state, energy and phase transformations.
Scope
This area covers the thermodynamic behaviour of dry and moist air: the gas laws and hydrostatic balance, the first law applied to ascending and descending parcels, adiabatic processes and lapse rates, the thermodynamics of phase changes of water, static stability and the energetics of convection, and the graphical thermodynamic diagrams used to analyse atmospheric soundings.
Sub-topics
Core questions
- How does a rising air parcel cool, and how does moisture change this rate?
- What determines whether the atmosphere resists or favours vertical motion?
- How is the energy available for convection diagnosed from a temperature and humidity profile?
Key theories
- First law applied to air parcels
- Treating an air parcel as a thermodynamic system relates its temperature changes to work done during expansion and to latent heat released by condensation, yielding the dry and moist adiabatic lapse rates.
- Parcel theory of stability
- Comparing a displaced parcel's temperature with its environment determines buoyancy and hence static stability, the basis for diagnosing convection.
Mechanisms
Air closely obeys the ideal gas law, and its vertical pressure structure follows hydrostatic balance. As a parcel rises it expands and cools at the dry adiabatic rate of about 9.8 degrees Celsius per kilometre until saturation, after which latent heat release reduces cooling to the moist adiabatic rate. The difference between a parcel's temperature and that of its surroundings sets its buoyancy, controlling whether vertical displacements are damped or amplified.
Clinical relevance
Thermodynamic reasoning underlies forecasting of thunderstorms and severe convection, the construction of model physics, and the interpretation of radiosonde soundings.
History
Atmospheric thermodynamics matured in the late nineteenth and early twentieth centuries as Hertz, von Bezold and Normand applied classical thermodynamics to moist air, producing the adiabatic chart and the concept of equivalent potential temperature still central to convective analysis.
Key figures
- Craig Bohren
- Julio Iribarne
Related topics
Seminal works
- bohren1998
- iribarne1981
Frequently asked questions
- Why does rising air cool even without losing heat to its surroundings?
- A rising parcel expands against lower surrounding pressure, doing work and converting internal energy into work, so its temperature falls adiabatically even with no heat exchange.