{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"11a544fc3c104f610dfe30f9de859478bd0877b5a5d417ac78109b645aa3d0e6","created":"2026-10-03T07:18:02Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["4ca8b0a0652f0998e907c36cae2200f6310b9fe6a7760c07a3f135f76921930c","5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"c6a4c3f5b29a8434bfc42fb3d0feec8290396efe139890963a9b566eec719650","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"earth.atmospheric-electricity.cloud-charging","kind":"phenomenon","literature_status":"open","n":"1","parents":[{"note":"Graupel-ice collisions are same-material contact charging with temperature-dependent sign","parent_revision":"4ca8b0a0652f0998e907c36cae2200f6310b9fe6a7760c07a3f135f76921930c","relation":"special_case"}],"plain":"Thunderclouds become charged when small ice crystals bounce off soft hail pellets (graupel), each carrying away opposite charge. Why the charge goes one way or the other, and why its sign flips with temperature and cloud water content, is not understood at the molecular level.","posed_since":"","precise":"Laboratory experiments since the 1970s show that rebounding collisions between ice crystals and riming graupel transfer charges of order $3e-6 \\text{ to } 3e-4\\,\\mathrm{statC}$ per collision, with the sign of graupel charge reversing near $253 \\text{ to } 263\\,\\mathrm{K}$ depending on liquid water content (review: Saunders, Space Science Reviews 2008, https://doi.org/10.1007/s11214-008-9345-0). Identify the microphysical mechanism (relative vapor growth rates of the colliding surfaces, mass transfer through the quasi-liquid surface layer, ionic impurities) and derive a charge-transfer law as a function of temperature, liquid water content, impact speed and particle size. An answer is a physical law that reproduces the laboratory charging diagrams and the observed tripole charge structure of storms.","problem_ref":null,"references":"","settled_by":"Single-collision experiments with controlled surface growth rates and impurity content compared with a molecular-level model of charge transfer between ice surfaces.","status_note":"","title":"How do collisions between ice particles charge 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