This talk will examine the physics of drizzle formation. Although not in itself a phase transformation process, drizzle formation is shown to be analogous to and quantitatively described using homogeneous nucleation theory. Specifically, the kinetic potential approach to nucleation is introduced and used to describe the complicated
droplet growth processes in a cloud.

Drizzle formation is identified as a statistical barrier-crossing phenomenon that transforms cloud droplets to drizzle size with a rate dependent on turbulent evaporation and growth, droplet
collection efficiency, and cloud liquid water content and droplet number concentration.

The talk will review important nucleation and growth processes of atmospheric aerosols and clouds, as well as methods from modern homogeneous nucleation theory. The effects of aerosols on cloud droplet distribution, cloud optical properties,
and drizzle formation will be discussed.

The new drizzle model is shown to quantify an important indirect effect of aerosols on climate – suppression of rain in clouds of higher droplet concentration. A parameterization based
on the new model is presented and compared with observations from the recent MArine Stratus Experiment (MASE).