Coefficient of restitution as a function of relative impact velocity. a) Relative impact velocity versus coefficient of restitution. The dashed lines show the limits of coefficient of restitution for this data and for collisions of 1.5 cm ice spheres in the velocity range 0.06-0.22 m s^-1 (found by Heißelmann et al. 2010). b) Relative impact velocity versus coefficient of restitution tangential to the colliding surfaces. c) Relative impact velocity versus coefficient of restitution normal to the colliding surfaces.