The Twomey Effect: more seeds, smaller droplets

Same liquid water. Change only the number of cloud condensation nuclei. Watch the spectrum shift.

CCN (cm³) 100
1.0
Mean radius r ≈ — μm
Spread: broad
Rain likelihood: favorable
Clean maritime • ~100 CCN cm⁻³
Average droplet
15 μm
Spectrum
8–25 μm
Selected air • ~2000 CCN cm⁻³
Average droplet
7 μm
Spectrum
6–9 μm
Fundamental relationship for warm clouds: r ∝ (w / N)^(1/3), where r is droplet radius, w is available liquid water, and N is CCN count. Increasing N narrows the size spectrum and shifts it smaller, which hampers collision–coalescence.