Five years ago, scientists reported that raindrops, especially small ones, often fall through the air much faster than they should. Some researchers have suggested that these “super-terminal” raindrops (ones traveling more than 30% faster than their terminal velocity, at which air resistance prevents further acceleration due to gravity) were fragments of larger drops that had splattered off the team’s instruments, with the smaller bits retaining the speed the larger drop had before it struck the instrument. But new research hints that the speedier-than-expected drops are the result of natural processes—and that, moreover, they make up a substantial fraction of rainfall. In the new study, scientists used 22 instruments typically used to monitor precipitation to measure the falling speeds of more than 1.5 million raindrops during six different storms at a site near Charleston, South Carolina, over a 5-month period. Whereas all drops 0.8 millimeters and larger fell at expected speeds, between 30% and 60% of those measuring 0.3 mm dropped at super-terminal speeds, the researchers report online ahead of print in Geophysical Research Letters. It’s not yet clear why these drops are falling faster than expected, the researchers say. But according to one notion, the speedy drops are fragments of larger drops that have broken apart in midair but have yet to slow down. If that is indeed the case, the researchers note, then raindrop disintegration happens normally in the atmosphere and more often than previously presumed—possibly when drops collide midair or become unstable as they fall through the atmosphere. Further study could improve estimates of the total amount of rainfall a storm will produce or the amount of erosion that it can generate.
Confirmed: Some raindrops fall faster than they should
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