How do cloud droplets cross the size gap to form warm rain?
In plain words
Cloud droplets grow quickly by condensation while tiny and by sweeping up others once large, but in between, at a few tens of micrometers, both processes are slow. Yet rain appears within about 20 minutes, and which process bridges the gap is disputed.
Precise statement
In warm (ice-free) cumulus, droplets grow by condensation to about 10-15 micron radius and by gravitational collection above about 40 micron, with slow growth in between, yet rain appears 15-30 min after cloud formation. Identify which process (turbulent collision enhancement, giant aerosol nuclei, entrainment mixing, lucky-droplet statistics, electrostatic forces) dominates growth across the gap for typical cloud dissipation rates $\epsilon \sim 10\text{ to }1000\,\mathrm{cm}^2/\mathrm{s}^3$.
What would settle it
In-cloud measurements of droplet size distributions and turbulence compared with collision-coalescence simulations that include each candidate process with measured inputs.
Status in the literature
Unverified note
A 2024 kinetic analysis places the low-efficiency gap at about 3-30 micron and argues the turbulence pathway is unlikely (Poydenot and Andreotti, arXiv:2410.11072), against DNS reports of strong turbulent enhancement.