A scientific study of raindrops has found the weathermen have been wrong for decades and have reversed the theory that large raindrops fall faster than smaller drops.
Meteorologists have always assumed large raindrops fall from the sky faster because they're bigger and heavier.
But following a three-year study of 64,000 raindrops, a team of researchers have finally determined that smaller raindrops actually fall quicker and in some cases faster than “terminal speed”.
And they say their findings prove the weather forecasters have been using the wrong formulas to predict rainfall and have been overestimating the amount it will rain.
It has always been accepted that a raindrop cannot fall faster than its “terminal speed”, which is the speed when the downward force of gravity is exactly the same as the upward air resistance.
But a joint team of physicists from Michigan Technological University and National University of Mexico have discovered that small raindrops do fall faster than bigger ones and at a rate quicker than drops that size and weight are supposed to be able to fall.
The researchers' findings to be published in the US science journal Geophysical Research Letters will improve the accuracy of weather measurement and prediction.
The researchers gathered data during natural rainfalls at the Mexico City campus of the National University of Mexico. They studied approximately 64,000 raindrops over three years, using optical array spectrometer probes and a particle analysis and collecting system.
By using the high-tech measuring equipment they found clusters of raindrops falling faster than their terminal speed, and as the rainfall became heavier, they recorded more and more of the unexpectedly speedy drops.
They think that the "super-terminal" drops come from the break-up of larger drops, which produces smaller fragments all moving at the same speed as their parent raindrop and faster than the terminal speed predicted by their size.
"In the past, people have seen indications of faster-than-terminal drops, but they always attributed it to splashing on the instruments," Professor Raymond Shaw of Michigan Tech explained.
"Existing rain models are based on the assumption that all drops fall at their terminal speed, but our data suggest that this is not the case.” Shaw argued, saying that if rainfall is measured based on that assumption, large raindrops that are not really there will be recorded.
"If we want to forecast weather or rain, we need to understand the rain formation processes and be able to accurately measure the amount of rain," he added.
Taking super-terminal raindrops into account could be of real economic benefit, even if it leads only to incremental improvements in rainfall measurement and forecasting.
Approximately one-third of the economy—including agriculture, construction and aviation—is directly influenced by the ability to predict precipitation accurately. "And one-third of the economy is a very large sum of money, even during a recession," Shaw said.
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