{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"5ed66aef3c551575d0b440a41bd3329762b4c879b3d1454147e23cbecf17e307","created":"2026-10-03T07:18:03Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"1efc551b3f43957fd896b51ad25ab278829f690a4d8a37835b320ef14689b8b0","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"identification","assisted_by":[],"external_id":"fluid.particle-turbulence.rain-size-gap","kind":"phenomenon","literature_status":"contested","n":"1","parents":[],"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.","posed_since":"","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 $\\epsilon \\sim 10\\text{ to }1000\\,\\mathrm{cm}^2/\\mathrm{s}^3$.","problem_ref":null,"references":"","settled_by":"In-cloud measurements of droplet size distributions and turbulence compared with collision-coalescence simulations that include each candidate process with measured inputs.","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.","title":"How do cloud droplets cross the size gap to form warm rain?","topic_ref":"28a67e06c2d95a104488e4da0be70a03916f05f9e68b223b2403a21e1c4f018e"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"6e0f350309f814f4e1d69e9bf842015cff29b64a78fbbef2ce720a699d2c40b9","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"ff2683f9b6af0532e89adac49055b689965be56adc484726dfa869b79447cb70","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"8q8j_WuvHeRoyglT3XDvTA5s1oVte833R46NEDEHmbcq3Lpbz8WZWUOT7sBw5VQvUgE9dsheXJbpc2jumpD-CQ"},"schema":"pubphys.envelope/1"},"record_hash":"6e0f350309f814f4e1d69e9bf842015cff29b64a78fbbef2ce720a699d2c40b9","leaf_index":1416}