Warm Rain and Collision-Coalescence

How rain forms in clouds warmer than freezing through the collision and merging of liquid droplets.

Definition

The warm rain process is the formation of precipitation in clouds above the freezing level by the gravitational collision and coalescence of liquid water droplets into raindrops.

Scope

Covers the limits of condensational growth, the terminal fall speeds of droplets, collision efficiency and coalescence efficiency, the continuous and stochastic collection equations, the broadening of droplet size distributions, and the conditions under which warm clouds produce rain.

Core questions

• Why is condensation alone too slow to produce raindrops?
• How do differences in fall speed allow larger droplets to collect smaller ones?
• What determines whether a warm cloud will rain?

Key theories

Collision-coalescence growth

Larger droplets fall faster and sweep up smaller ones, growing by collection at a rate that accelerates with size and explains the rapid jump from cloud droplets to raindrops.

Mechanisms

Condensational growth narrows the droplet spectrum and stalls at drizzle sizes because the rate falls as droplets enlarge. Precipitation requires collection: larger droplets with higher terminal velocities overtake and collide with smaller droplets, and a fraction of collisions result in coalescence. The collection process is self-accelerating, so once some droplets reach a critical size of a few tens of micrometres they grow rapidly into raindrops. Stochastic variations in collection help broaden the spectrum and trigger this onset.

Clinical relevance

Warm-rain physics governs precipitation from tropical and maritime clouds and is central to evaluating the susceptibility of clouds to aerosol pollution and to warm-cloud seeding.

History

The recognition that collision and coalescence, rather than condensation, produces rain in warm clouds developed through the mid-twentieth century, with the stochastic collection framework refining earlier continuous-growth models.

Key figures

• Roddy Rogers
• Hans Pruppacher

Seminal works

• rogers1989
• pruppacher1997

Frequently asked questions

Why can't condensation alone make rain?

Condensational growth slows as droplets enlarge and would take far too long to reach raindrop size; collision and coalescence provide the much faster growth needed to produce rain.