Cloud-droplet size distribution ($n(r)$)

The number of cloud droplets per volume in each radius interval, often represented by $n(r)dr$. Its width controls differential terminal speeds and collection opportunities; condensational growth with a common $d(r^2)/dt$ narrows relative radius differences.

The ratio of the third to the second radius moment determines projected extinction area per liquid mass. For size-independent extinction efficiency $Q$, the local particle extinction coefficient is $3Qw/(4{\rho }_w{r}_e)$. It is not generally the mean radius or the mean-volume radius. In adiabatic cloud droplet moments, the shape factor gives $r_e=r_v/k^{1/3}$.