Cloud-droplet collision kernel ($K(R,r)=\pi (R+r{)}^2|v(R)-v(r)|E_{\mathrm{coll}}{E}_{\mathrm{coal}}$)

A gravitational collision kernel multiplies geometric encounter area, relative fall speed, collection efficiency and coalescence efficiency. Its units are volume per time. The number of encounters also depends on the droplet size distribution. Hydrodynamic deflection, breakup and turbulence change the simple settling picture. Condensational growth alone tends to reduce relative size differences, so a precipitation-growth calculation must explain how efficient collectors arise.