title: Caustic activation of rain showers.
abstract: We show quantitatively how the collision rate of droplets of visible moisture in turbulent air increases very abruptly as the intensity of the turbulence passes a threshold, due to the formation of fold caustics in their velocity field. The formation of caustics is an activated process, in which a measure of the intensity of the turbulence, termed the Stokes number St, is analogous to temperature in a chemical reaction: the rate of collision contains a factor exp(-C/St). Our results are relevant to the long-standing problem of explaining the rapid onset of rainfall from convecting clouds. Our theory does not involve spatial clustering of particles.
authors.affiliation: Faculty of Mathematics and Computing, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom.
authors.forename: Michael
authors.initials: M
authors.lastname: Wilkinson
authors.forename: Bernhard
authors.initials: B
authors.lastname: Mehlig
authors.forename: Vlad
authors.initials: V
authors.lastname: Bezuglyy
mesh_terms: D015984:Causality; D003198:Computer Simulation; D044085:Microfluidics / Q000379:methods*; D008956:Models, Chemical; D009038:Motion; D011891:Rain / Q000737:chemistry*