Caustic activation of rain showers

Phys Rev Lett. 2006 Jul 28;97(4):048501. doi: 10.1103/PhysRevLett.97.048501. Epub 2006 Jul 28.

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.

MeSH terms

  • Causality
  • Computer Simulation
  • Microfluidics / methods*
  • Models, Chemical*
  • Motion
  • Rain / chemistry*