  The evolution of a single raindrop falling below a cloud is governed by fluid
dynamics and thermodynamics fundamentally transferable to planetary atmospheres
beyond modern Earth's. Here, we show how three properties that characterize
falling raindrops -- raindrop shape, terminal velocity, and evaporation rate --
can be calculated as a function of raindrop size in any planetary atmosphere.
We demonstrate that these simple, interrelated characteristics tightly bound
the possible size range of raindrops in a given atmosphere, independently of
poorly understood growth mechanisms. Starting from the equations governing
raindrop falling and evaporation, we demonstrate that raindrop ability to
vertically transport latent heat and condensible mass can be well captured by a
new dimensionless number. Our results have implications for precipitation
efficiency, convective storm dynamics, and rainfall rates, which are properties
of interest for understanding planetary radiative balance and (in the case of
terrestrial planets) rainfall-driven surface erosion.
