Some Of The Biggest Raindrops On Record Found In Both Clean And Dirty Air

On two occasions, separated by four years and thousands of miles and in very different conditions, raindrops were measured at sizes similar to or greater than the largest ever recorded. The largest ones were at least 8 millimeters in diameter, about the same as the largest previously observed, and were possibly a centimeter – about four-tenths of an inch or one-fourth the diameter of a golf ball – according to findings published today in the online edition of Geophysical Research Letters, a journal of the American Geophysical Union.

As they fall toward earth, raindrops don't have the cliché teardrop shape, said Peter Hobbs, a UW atmospheric sciences professor. Instead, they are shaped more like a parachute, or a jellyfish.

"Most of the water is around the rim of the drop, while the upper part of the drop is a very thin film of water," Hobbs said.

When air forces through the thin film at the top, large drops break up into many smaller drops. Large drops also can break up when they collide with other drops. In laboratory conditions, breakup usually happens when the drops reach about 5 millimeters, or about one-fifth of an inch, in diameter. In clouds, when drops reach that size they generally break up in collisions with other drops.

"That's why it's so rare to see a raindrop of 5 millimeters or more on the ground, because it would mean the drop had avoided collisions with the many other drops in a cloud," Hobbs said.

Yet the UW Cloud and Aerosol Research Group recorded raindrops of at least 8 millimeters, and perhaps a centimeter, during research flights through cumulus congestus clouds spawned by a burning forest in Brazil in 1995. Giant raindrops also were recorded during a flight through cumulus clouds in clean marine air over the Marshall Islands in 1999.

Hobbs and Arthur Rangno, a UW atmospheric sciences research meteorologist, are co-authors of the paper documenting the findings. The work was supported by funding from the National Science Foundation and the National Aeronautics and Space Administration.

The scientists speculate the giant raindrops in the polluted air above the burning Amazon forest could have formed around large ash particles, while those in the Marshall Islands might have been spawned by particles of sea salt.

It is not unusual to see large drops in clouds formed in clean marine air because there are fewer particles on which the water can collect, Hobbs said. But smoky air typically produces small raindrops because there are so many particles for the water to condense upon, so it is unclear why the huge raindrops formed over Brazil. He noted that collisions of drops can produce larger drops that then gather up additional small drops as they fall, much as a drip does as it slides down a window pane. That could be what happened in the Brazil storm clouds, as a few favored raindrops fell through, and collected, smaller drops present in large concentrations within particular regions of the clouds.

The scientists discovered the giant raindrops as they culled mountains of data recorded when the research group still operated its own aircraft. The aircraft was sold in 2002, but scientists still are going through data from a number of missions, including campaigns in Africa and the United States, as well as Brazil and the South Pacific.

"We collected so much data on those flights, recording thousands of events a minute, and it will take years to analyze all of the information," Hobbs said. "We just discovered the giant drops by accident when we were looking for something else."