Raindrops Achieve Super-Terminal Velocity
Researchers have confirmed the existence of raindrops that fall at super terminal speeds. Accounting for these unusually speedy drops could actually help increase the accuracy of rainfall and erosion estimates.
You’d think scientists would have a real good handle by now on how much rain is falling, not to mention how fast a simple raindrop falls. That’s part of what make science so awesome though. Even the seemingly most straightforward of phenomena can contain hidden surprises. My favorite example was the discovery of micro-organisms when a microscope first peered at a simple drop of water.
In this case, there have been hints in the past that some small raindrops appear to fall abnormally fast when tested in experiments. This weird behavior though was attributed not to some unknown process in nature but to larger drops interacting with the testing machinery. They assumed that when these larger drops (which normally fall faster than smaller ones), hit the test machines, they split into smaller drops which contributed to the ostensibly erroneous results.
New research however seems to show that the instruments had nothing to do with the rain traveling at super terminal velocity. This was determined by researchers at the National Autonomous University of Mexico by measuring the shadows of raindrops as they passed through a beam of infrared light. After recording a whopping 1.5 million drops from six different storms, they determined that those drops that were larger than a millimeter fell at speeds that would be expected. Drops however that were around one third of a millimeter frequently exceeded the expected velocity, some by as much 10 times. These are the super terminal velocity drops.
So what is super terminal velocity?
You’ve probably heard the expression “terminal velocity”. That’s the maximum speed that a falling object can attain due to a battle between gravity and air resistance. Gravity tries to continue accelerating an object but air resistance is concurrently trying to slow it down. Where those two forces balance out is the terminal velocity. The “super” variety of terminal velocity just means that an object is falling more than 30% faster than that.
How is that possible?
Well the laws of physics are not being violated if that’s what you’re thinking. The drops aren’t creating a hyperspace tunnel to bypass physics as we know it. The researchers aren’t exactly sure yet how this is accomplished but most suspect that bigger, faster moving drops in the atmosphere are fragmenting and temporarily imparting their velocity to their smaller offspring. This may happen due to drops colliding or perhaps due to the instability inherent in a falling drop. These drops aren’t perfect spheres after all, nor are they tear-drop shaped either. While in freefall, they are somewhat flattened on the bottom and somewhat wavy on top.
This new insight into falling rain may prove to be quite beneficial in terms of the impact a given storm may have. Currently, the radar used to assess rainfall as it’s happening does this by determining the speed of falling raindrops. Fast drops are assumed to be big. If many fast drops are detected, then the amount of rainfall and erosion can be calculated for a particular storm. If, however, small drops can be temporarily fast, then we may be overestimating the amount of rain because many of the big drops are actually small. Some say this overestimate may be by as much as 20 percent. Others however like Professor Ana Barros of Duke University in North Carolina, says that current estimates are unlikely to be too far off since the small drops would quickly slow down to reach their true terminal velocity.
Image Credit: Image: Alistair McClymont