A new study published in the Journal of Geophysical Research: Planets is a first step toward a generalized theory of how precipitation and condensible cycles operate in planetary conditions different from modern Earth.
Liquid precipitating particles are remarkably similar across different planetary atmospheres. Image credit: David A. Hardy, AstroArt / NASA.
The behavior of clouds and precipitation on planets beyond Earth is poorly understood, but understanding clouds and precipitation is important for predicting planetary climates and interpreting records of past rainfall preserved on the surfaces of Earth, Mars, and Titan.
One component of the clouds and precipitation system that can be easily understood is the behavior of individual liquid precipitating particles (raindrops).
“The lifecycle of clouds is really important when we think about planet habitability,” said Kaitlyn Loftus, a graduate student in the Department of Earth and Planetary Sciences at Harvard University.
“But clouds and precipitation are really complicated and too complex to model completely.”
“We’re looking for simpler ways to understand how clouds evolve, and a first step is whether cloud droplets evaporate in the atmosphere or make it to the surface as rain.”
“The humble raindrop is a vital component of the precipitation cycle for all planets,” said Dr. Robin Wordsworth, a researcher at the Harvard John A. Paulson School of Engineering and Applied Sciences.
“If we understand how individual raindrops behave, we can better represent rainfall in complex climate models.”
In their paper, Loftus and Dr. Wordsworth show how to calculate three key properties that characterize raindrops: their shape, their falling speed, and the speed at which they evaporate.
“Drop shapes are the same across different rain materials and primarily depend on how heavy the drop is,” they explained.
“While many of us may picture a traditional tear-shaped droplet, raindrops are actually spherical when small, becoming squashed as they grow larger until they transition into a shape like the top of a hamburger bun.”
“Falling speed depends on this shape as well as gravity and the thickness of the surrounding air.”
“Evaporation speed is more complicated, influenced by atmospheric composition, pressure, temperature, relative humidity and more.”
By taking all of these properties into account, the researchers found that across a wide range of planetary conditions, the math of raindrop falling means only a very small fraction of the possible drop sizes in a cloud can reach the surface.
“We can use this behavior to guide us as we model cloud cycles on exoplanets,” Loftus said.
“The insights we gain from thinking about raindrops and clouds in diverse environments are key to understanding exoplanet habitability,” Dr. Wordsworth added.
“In the long term, they can also help us gain a deeper understanding of the climate of Earth itself.”
K. Loftus & R.D. Wordsworth. 2021. The physics of falling raindrops in diverse planetary atmospheres. Journal of Geophysical Research: Planets 126 (4): e2020JE006653; doi: 10.1029/2020JE006653