  Aggregation of ice crystals is a key process governing precipitation.
Individual ice crystals exhibit considerable diversity of shape, and a wide
range of physical processes could influence their aggregation; despite this we
show that a simple computer model captures key features of aggregate shape and
size distribution reported recently from Cirrus clouds. The results prompt a
new way to plot the experimental size distributions leading to remarkably good
dynamical scaling. That scaling independently confirms that there is a single
dominant aggregation mechanism at play, albeit our model (based on undeflected
trajectories to contact) does not capture its form exactly.
