The smaller the drop the faster the rain, C of C research says

Field equipment measures rainfall for College of Charleston at Dixie Plantation in Hollywood.

Very tiny rain falls very fast — faster in fact than the laws of physics say it should. How, you ask? Well, it’s a head scratcher for sure.

But the findings of field tests recently conducted at the College of Charleston could improve how forecasters read weather radar, predict storms or determine erosion rates. Raindrops are different sizes, some drops smaller than a pencil tip. Those guys were just proven to exceed terminal velocity — the point at which air friction should slam the brakes on the acceleration caused by gravity.

The study confirmed an earlier finding. The “faster drops” phenomenon is enough of a puzzle that part of the job of the field tests at Dixie Plantation in Hollywood was to show that the first readings weren’t just “bad bounces” off the measuring equipment.

But it’s for real. Raindrops one millimeter or smaller drop faster than the 3 mph or so they should, said Michael Larsen, physics professor. Why? “The real answer is, nobody knows,” he said. The three best guesses are:

Larger drops splitting and their speed getting measured before air friction slows them down.

A thrust of air pressure pushing them down, sort of like the sudden drop sometimes experienced by aircraft in turbulence.

The “wake effect,” of drops riding the slipstream of bigger drops falling before them, sort of like a NASCAR racer drafting the car ahead.

Whatever the answer turns out to be, the finding data could plug some pretty big holes in computer modeling rainfall.

“The density and size of rain is so complex. Every raindrop is unique,” said senior forecaster Richard Thacker, National Weather Service, Charleston. That makes calculating rainfall so problematic that the algorithms must be adjusted for tropical storms, and “still tend to not be perfect,” he said. Better data can only help.

Michigan Tech researchers who first measured the phenomenon asked Larsen to test the finding at the college’s field study lab, an array of poles and gauges at the plantation, that is considered the densest of its type in the world. They were looking for proof, and they got it.

“We’re learning a lot about how rain varies over space and time scales,” Larsen said.

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