A pair of scientists at Harvard recently published a paper in the Journal of Geophysical Research: Planets examining the physics of raindrops and how they might work in different atmospheres. Lead author Robin Wordsworth explains: The humble raindrop is a vital component of the precipitation cycle for all planets. If we understand how individual raindrops behave, we can better represent rainfall in complex climate models.

And it turns out? Raindrops are pretty much raindrops no matter where they fall, although whether or not they reach the surface is a matter of size: too big and the surface tension isn’t enough to hold drops together; too small and the drop evaporates in the atmosphere. What makes a drop “just right”? Well, here you have to combine a few properties — drop shape, falling speed, and evaporation speed. 

Drop shapes are pretty similar no matter the material: spherical when small and then squashed flat as they grow. The falling speed is basically a question of the planet’s gravity and the density of the atmosphere. Evaporation speed is the trickstery factor, but taking all that into account led Wordsworth and grad student Kaitlyn Loftus to conclude that no matter the conditions, only a small fraction of drops hit the surface. Again, understanding Earth can help us find the habitable exoplanets we’re looking for, at least so far as life as we know it is considered.

More Information

Harvard press release

AGU press release

“The Physics of Falling Raindrops in Diverse Planetary Atmospheres,” Kaitlyn Loftus  Robin and D. Wordsworth, 2021 March 15, JGR: Planets