Above about four kilometers up in the atmosphere, clouds begin to contain both liquid droplets and ice crystals. Because two phases of water coexist, a variety of interesting small-scale processes can occur in the cloud: droplets can coalesce and ice crystals can aggregate or droplets can collide with ice crystals and freeze or ice crystals can melt as they sediment out of the cloud. These processes are jointly called cloud microphysics and can be quite difficult to model. Ice crystals...