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Raindrops keep falling -- too quickly -- on my head

Bigger water droplets should fall faster than small ones, but that isn't always the case. Understanding why could help researchers improve weather forecasting.

June 20, 2009|John Johnson Jr.

The hows and whys of rainfall have been pondered for about as long as human beings have walked upright. Now a research team thinks it has discovered something everyone else missed: Some raindrops fall faster than they should.

Curious though it is, the finding could have consequences far beyond inspiring water cooler talk at the National Weather Service.

According to the researchers at Michigan Technological University and the National University of Mexico, their exhaustive study of how tiny drops of rain behave as they tumble out of the sky, bumping and mixing and breaking apart from their soggy brethren, could ultimately be used to improve weather forecasting.

Meteorologists build weather models with the assumption that the largest raindrops fall at the greatest speeds and hit the ground with the most force. That's because the speed ultimately reached by a raindrop (or any falling object) is related to its mass.

The falling drop increases speed until the resistance of the air equals the pull of gravity, at which point the drop begins to fall at a constant speed, its terminal velocity.

"Just as an adult will fall faster than a child, heavier raindrops fall faster than smaller ones," said physics professor Alexander Kostinski of Michigan Tech in Houghton, Mich.

No raindrop can exceed its terminal velocity. Or so scientists thought. After studying 64,000 individual raindrops over three years at the Mexico campus, the researchers found something different.

"Many drizzle-sized fragments move a lot faster than they are supposed to," Kostinski said.

The scientists figured out that some of the raindrops that hit the ground at less than 2 millimeters in diameter were in fact the remains of a much bigger drop that broke up as it fell.

When the small drops split off, "the fragments can start out falling at the same speed as the parent drop," Kostinski said. "That may be up to 10 times faster than the smaller drop's terminal velocity." When the fragments hit the ground, they are still traveling faster than expected.

Kostinski used the analogy of someone throwing a bottle from a train to explain the process. If the train is standing still, the bottle flies a lot slower than if the train is speeding along at 60 miles an hour.

The finding, published last week in Geophysical Research Letters, could affect weather forecasting. Some forecasters use an instrument called a disdrometer, which estimates rainfall based on the force with which drops hit a measuring plate.

If smaller drops land with the same force as bigger drops, the instrument could assume more rain is falling than actually is, the researchers said.

"If we want to forecast weather or rain, we need to . . . be able to accurately measure the amount of rain," Raymond Shaw, another of the researchers, said in a statement.

Rob Hartman of the weather service's River Forecast Center in Sacramento said the team's findings wouldn't have much effect on rainfall measurements in cities, such as Los Angeles, that use conventional rain collection systems to estimate rainfall.

However, he added, a disdrometer is being used in a test project on the American River "to improve the ability to forecast precipitation."


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