Lecturer: Michael Wilkinson, Department of Mathematics and Statistics, The Open University, Walton Hall, Milton Keynes, MK7 6AA, England Abstract: Rainfall from ice-free cumulus clouds requires collisions of very large numbers of microscopic droplets to create every raindrop. The collision rate for the first few droplet coalescences is typically less than one per hour, so it appears surprising that rain showers can happen at all. I use show how ideas from large-deviation theory demonstrate that the onset of rain showers can be surprisingly rapid, much faster than the mean time required for a single collision. I also discuss whether these results apply to planet formation. If planets grow by accretion of dust particles, this must happen very rapidly because objects of roughly metre size spiral into the star over a timescale of less than a thousand years. For this problem, large deviation theory is less promising. I discuss an alternative approach. ‏האירוע הזה כולל שיחת וידאו ב-Google Hangouts. הצטרף: https://hangouts.google.com/hangouts/_/mail.huji.ac.il/physics?hceid=bWF...