You might expect the physics of rain to be something that is well understood by now, but sometimes raindrops fall faster than our current formulas say should be possible. Previous measurements indicating this fact have been confirmed and new data demonstrates just how drastically theoretical speed limits are being broken.
In a vacuum, entities fall faster and faster under the influence of gravity. However, objects that fall through the air will reach a terminal velocity, where the resistance of the air balances the force of gravity. Calculations of terminal velocity based on weight and surface area are staples of undergraduate physics courses. For roughly spherical objects of consistent density, terminal velocity rises with size, since weight increases faster than surface area.
A scientific collaboration disrupted this simple picture in 2009 by measuring raindrops falling faster than should have been possible for their size. Particularly puzzling was the fact that not all raindrops did this. Instead, in a sample of 64,000 drops, “superterminal” clusters were identified, with intermediate-sized raindrops observed breaking speed limits when the rain was heavy.
Nor were the drops just edging past the theoretical maximum in a way that might be explained with a slight tweaking of equations. Some drops were measured doing ten times terminal velocity; try explaining doing 600 in a 60 zone to the police.
At the time, the researchers concluded the superterminal drops were “fragments of a recent break-up, moving with the speed of the parent drop.” Since terminal velocity is the end speed of a sufficiently long fall, whether the starting speed is faster or slower, the expectation was that the drops would eventually slow to the proper speed.
Professor Alexander Kostinski, a member of the original team that made the discovery, has now teamed up with two new colleagues to delve deeper. In Geophysical Research Letters, Kostinski of Michigan Technology University reveals that 30-60% of appropriately sized drops are superterminal, potentially requiring a new explanation.
Kostinski used 21 laser trackers and a video to measure the velocity of 1.5 million raindrops in six storms. All the superterminal drops observed were less than 0.8 millimeters across, but this time they were studied in all types of rain events.
“The fact that a substantial fraction of drizzle-sized drops are moving faster than their terminal velocities suggest that we are not just seeing an outlier effect here,” said lead author Dr. Michael Larsen of the College of Charleston.
It's possible that superterminal drops are the result of fragmenting from a "parent" droplet. As the paper note: "The mother drop is large, and its terminal speed is much higher than the one of smaller drops. This is one possible reason for smaller drops (fragments), breaking the speed limit.
An alternative explanation is the physicists' old favorite for misbehaving objects: turbulence. Kostinski speculates that the drops may be behaving like flying geese or road racing cyclists, using the reduced drag created by a leader to speed the movement. Where geese and cyclists are of similar size, however, these speeds would only be possible if drops are following in the wake of larger drops.
The findings matter because faster drops cause more erosion, potentially forcing a rewrite of storm damage models.