Dynamics of water droplets on a window pane

We performed experiments and computer simulations in order to investigate whether the rain drops on a window pane are in a ‘‘self-organized critical state,’’ as proposed by Bak, Tang, and Wiesenfeld [Phys. Rev. Lett. 59, 381 (1987); Phys. Rev. A 38, 364 (1988); J. Stat. Phys. 51, 797 (1988)]; and Tang and Bak [Phys. Rev. Lett. 60, 2347 (1988)]. In contrast to the expected behavior, we found that the coverage fluctuations exhibit an $f^{\mathrm{-}2}$ power spectrum, and the characteristic distribution functions of the elementary events have no power-law dependence.