Revision 1

Accepted · first revision · record

Title: How do cloud droplets cross the size gap to form warm rain?
Plain: 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: 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 $\epsilon \sim 10\text{ to }1000\,\mathrm{cm}^2/\mathrm{s}^3$.
Settled by: In-cloud measurements of droplet size distributions and turbulence compared with collision-coalescence simulations that include each candidate process with measured inputs.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: contested
Status 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.
Posed since: (empty)