Cloud and Precipitation Physics
The microphysical processes by which water vapour forms cloud droplets and ice crystals and grows into rain, snow and hail.
Definition
Cloud and precipitation physics is the study of the formation, growth and fallout of liquid and ice hydrometeors in the atmosphere at the scale of individual particles.
Scope
This area covers the nucleation of cloud droplets on aerosol particles, the growth of droplets by condensation and by collision and coalescence, the formation and growth of ice crystals, mixed-phase processes including riming and aggregation, the role of cloud condensation and ice nuclei, and the transition from cloud particles to precipitation.
Sub-topics
Core questions
- How do cloud droplets form on aerosol particles?
- By what processes do droplets and crystals grow large enough to fall as precipitation?
- How do ice and liquid coexist and interact in mixed-phase clouds?
Key theories
- Kohler theory of droplet activation
- Combining the curvature (Kelvin) and solute (Raoult) effects, Kohler theory predicts the critical supersaturation at which a haze particle activates into a growing cloud droplet.
- Collision-coalescence and the Bergeron process
- Precipitation forms either by larger droplets colliding and merging in warm clouds or by ice crystals growing at the expense of supercooled droplets in cold clouds.
Mechanisms
Water vapour condenses onto cloud condensation nuclei once supersaturation exceeds the critical value given by Kohler theory, producing a population of small droplets. These grow first by diffusion of vapour and then, more rapidly, by gravitational collision and coalescence. In colder clouds ice crystals nucleate on ice nuclei or by freezing of droplets and grow by vapour deposition in the presence of supercooled water (the Wegener-Bergeron-Findeisen process), by riming and by aggregation, eventually forming snow, graupel or hail.
Clinical relevance
Cloud microphysics governs precipitation efficiency, cloud radiative properties and aerosol-cloud interactions, making it central to weather prediction, climate sensitivity estimates and assessments of cloud seeding.
History
The ice-crystal theory of precipitation was proposed by Wegener and developed by Bergeron and Findeisen in the 1930s, while Kohler's 1936 theory explained droplet activation. The field was synthesized in Pruppacher and Klett's comprehensive monograph, a standard reference since the 1970s.
Key figures
- Tor Bergeron
- Hilding Kohler
- Hans Pruppacher
Related topics
Seminal works
- pruppacher1997
- rogers1989
Frequently asked questions
- Why do most raindrops in midlatitudes start as ice?
- In cold clouds the Wegener-Bergeron-Findeisen process lets ice crystals grow rapidly at the expense of supercooled droplets; the crystals fall, melt and reach the ground as rain.