Atmospheric Thermodynamics and Moisture
Heat and water are the fuel of weather: as air rises and cools, water vapor condenses and releases energy, powering the clouds, storms, and rain that thermodynamics lets us understand and quantify.
Definition
Atmospheric thermodynamics and moisture is the branch of meteorology concerned with the energy and water content of air, the processes of expansion, cooling, condensation, and evaporation that govern it, and their consequences for stability, clouds, and precipitation.
Scope
This area covers the thermodynamics of dry and moist air, the assessment of atmospheric stability and the convection it permits, the measurement and behavior of atmospheric humidity, and the microphysical processes by which clouds and precipitation form.
Sub-topics
Core questions
- How do the laws of thermodynamics apply to rising and sinking air?
- What determines whether the atmosphere is stable or prone to convection?
- How is the water-vapor content of air measured and expressed?
- How do cloud droplets and precipitation particles form and grow?
Key theories
- Adiabatic processes and lapse rates
- Rising air expands and cools at the dry adiabatic rate until saturation, after which latent-heat release slows the cooling to the moist adiabatic rate, the basis for assessing stability and cloud development.
- Cloud microphysics
- Cloud droplets form on aerosol nuclei and grow by condensation and collision, and through warm-rain or ice processes become large enough to fall as precipitation.
Mechanisms
When a parcel of air rises it expands and cools without exchanging heat with its surroundings, an adiabatic process; once it cools to saturation, water vapor condenses and releases latent heat. Whether the parcel keeps rising depends on how its temperature compares with the environment, which defines stability and convection. The condensed water forms clouds, and microphysical growth by condensation, collision, and ice processes can produce particles heavy enough to fall as precipitation.
Clinical relevance
Thermodynamic and moisture principles underlie forecasts of cloud and precipitation, of thunderstorm potential through measures such as convective available potential energy, of fog and frost, and of comfort indices, making them indispensable to operational meteorology and aviation.
History
Nineteenth-century thermodynamics was applied to the atmosphere through the work on adiabatic processes and the development of the tephigram and other thermodynamic diagrams; in the twentieth century Bergeron, Findeisen, and others established the microphysics of cloud and precipitation formation that completes the field.
Key figures
- Tor Bergeron
- Walter Findeisen
- Hilding Kohler
Related topics
Seminal works
- wallace2006
- rogers1989
Frequently asked questions
- Why does rising air cool?
- As air rises into lower pressure it expands, and because expansion does work at the expense of the air's internal energy, the air cools even though no heat is removed, a process called adiabatic cooling.
- Why do clouds form when air rises?
- Rising air cools until it reaches saturation, the point at which it can hold no more water vapor; the excess vapor then condenses onto tiny particles to form the droplets or ice crystals that make up a cloud.