This paper by Saffman and Turner proposes a theory to explain collisions between small drops in turbulent clouds, considering both equal and unequal drops. The collision rates depend on the drop dimensions, energy dissipation rate ε, and kinematic viscosity ν. The authors argue that the collision efficiency for nearly equal drops is unity, and the collision rate due to spatial variations in turbulent velocity is given by \( N = 1.30 (r_1 + r_2)^9 n_1 n_2 (\epsilon / \nu)^{1/2} \), valid for \( r_1 / r_2 \) between one and two. Numerical integration shows how an initially uniform distribution changes due to collisions. The effects of gravity and turbulent accelerations on collisions between drops of different inertia are also considered. The theory is applied to small drops in atmospheric clouds to assess the importance of turbulence in initiating rainfall. Estimates of ε for typical conditions are made, and the initial collision rates, changes in mean properties, and production of large drops are calculated. The results suggest that turbulence in layer clouds has minimal impact, but moderate turbulence in cumulus clouds can broaden the drop size distribution in nearly uniform clouds. In heterogeneous clouds, collision rates increase, and the effects of drop inertia become significant. Turbulence-induced collisions between unequal drops become comparable to gravity-induced collisions when ε is about 2000 cm² sec⁻³.This paper by Saffman and Turner proposes a theory to explain collisions between small drops in turbulent clouds, considering both equal and unequal drops. The collision rates depend on the drop dimensions, energy dissipation rate ε, and kinematic viscosity ν. The authors argue that the collision efficiency for nearly equal drops is unity, and the collision rate due to spatial variations in turbulent velocity is given by \( N = 1.30 (r_1 + r_2)^9 n_1 n_2 (\epsilon / \nu)^{1/2} \), valid for \( r_1 / r_2 \) between one and two. Numerical integration shows how an initially uniform distribution changes due to collisions. The effects of gravity and turbulent accelerations on collisions between drops of different inertia are also considered. The theory is applied to small drops in atmospheric clouds to assess the importance of turbulence in initiating rainfall. Estimates of ε for typical conditions are made, and the initial collision rates, changes in mean properties, and production of large drops are calculated. The results suggest that turbulence in layer clouds has minimal impact, but moderate turbulence in cumulus clouds can broaden the drop size distribution in nearly uniform clouds. In heterogeneous clouds, collision rates increase, and the effects of drop inertia become significant. Turbulence-induced collisions between unequal drops become comparable to gravity-induced collisions when ε is about 2000 cm² sec⁻³.