Cloud Droplet Nucleation

How water vapour condenses onto aerosol particles to form the initial population of cloud droplets.

Definition

Cloud droplet nucleation is the process by which water vapour condenses onto an aerosol particle and the resulting solution droplet grows past a critical size to become an activated cloud droplet.

Scope

Covers homogeneous and heterogeneous nucleation, the Kelvin curvature effect and the Raoult solute effect, Kohler theory and the critical supersaturation for activation, the dependence of activation on particle size and composition, and the role of hygroscopic aerosol as cloud condensation nuclei.

Core questions

• Why can clouds not form by spontaneous condensation in the atmosphere?
• How do curvature and dissolved solute compete to set the activation threshold?
• Which aerosol particles are most effective as cloud condensation nuclei?

Key theories

Kohler theory

Kohler theory combines the Kelvin effect, which raises the equilibrium vapour pressure over curved surfaces, with the solute effect, which lowers it, to predict the critical supersaturation at which a particle activates.

Mechanisms

Pure-water droplet formation is inhibited by the Kelvin effect, which makes the equilibrium vapour pressure over a tiny droplet very high. Atmospheric droplets instead form on hygroscopic aerosol particles whose dissolved solute lowers the equilibrium vapour pressure. The competition between these effects, described by the Kohler curve, gives a critical radius and critical supersaturation; particles that reach this point activate and grow freely as cloud droplets, while smaller haze particles remain in stable equilibrium.

Clinical relevance

Droplet activation links aerosol abundance to cloud droplet number and thus to cloud brightness and lifetime, the basis of the aerosol indirect effect on climate.

History

Hilding Kohler formulated the theory of equilibrium of solution droplets in 1936, providing the quantitative framework for droplet activation that remains the foundation of cloud nucleation studies.

Key figures

• Hilding Kohler
• Hans Pruppacher

Seminal works

• kohler1936
• pruppacher1997

Frequently asked questions

What is critical supersaturation?

Critical supersaturation is the peak of the Kohler curve; once the ambient supersaturation exceeds it, an aerosol particle activates and its droplet grows without bound rather than staying in equilibrium.