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Size of Raindrops Forecasts Weather

Patrick Gatlin with the instrument that measured the largest raindrop recorded in the U.S. In Gatlin’s hand are steel spheres used to calibrate it.

Patrick Gatlin with the instrument that measured the largest raindrop recorded in the U.S. In Gatlin’s hand are steel spheres used to calibrate it.

Photo by Michael Mercier/UAH

Dialing in the accuracy of satellite weather forecasting is the goal behind basic research into the size and shape of raindrops being done at The University of Alabama in Huntsville.

Patrick Gatlin, a UAH doctoral student who also works with NASA, says his studies — which involve measuring the height and width of raindrops using ground instruments — provide an accuracy baseline that is then scaled up to ground radar and then to satellite measurements.

“That’s really the whole purpose of measuring raindrops, is for remote sensing purposes,” Gatlin says. Scaling up accuracy from a small sensor on the ground to large sections of the Earth being observed from space requires very accurately calibrated instruments. “Our ability to correctly depict rainfall using a sensor in space is closely tied to knowing how precipitation varies, right down to the individual raindrop and snowflake size.” 

Gatlin is about to finish up a global study focusing just on very large raindrops 5 millimeters in size and larger. These drops are difficult to capture in the small measuring area afforded by measuring instruments, so their observation is rare. Gatlin says out of 224 million drops he has researched, only 8,000 have been 5 mm or larger.

The largest drop collected thus far fell through a measuring device at the UAH campus. It measured 9.1 mm and was formed in a hailstorm when a falling piece of hail melted before landing.

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