Wednesday, June 10,
2009
It's conventional wisdom in atmospheric science circles: large
raindrops fall faster than smaller drops, because they're bigger
and heavier. And no raindrop can fall faster than its "terminal
speed"—its speed when the downward force of gravity is exactly the
same as the upward air resistance.
Now two physicists from Michigan Technological University and
colleagues at the Universidad Nacional Autónoma de México (National
University of Mexico) have discovered that it ain't necessarily
so.
Some smaller raindrops can fall faster than bigger ones. In fact,
they can fall faster than their terminal speed. In other words,
they can fall faster than drops that size and weight are supposed
to be able to fall.
And that could mean that the weatherman has been overestimating how
much it rains.
The findings of Michigan Tech physics professors Alexander
Kostinski and Raymond Shaw—co-authors with Guillermo
Montero-Martinez and Fernando Garcia-Garcia on a paper scheduled
for publication online June 13, 2009, in the American Geophysical
Union's journal Geophysical Research Letters—could 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. They also modified an algorithm or computational
formula to analyze the raindrop sizes.
They found clusters of raindrops falling faster than their terminal
speed, and as the rainfall became heavier, they saw more and more
of these 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," Shaw explains. He and his colleagues took special
precautions to prevent such interference, including collecting data
only during extremely calm conditions.
Their findings could significantly alter physicists' understanding
of the physics of rain.
"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 and Kostinski say. 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," Shaw pointed out.
Taking super-terminal raindrops into account could be of real
economic benefit, even if it leads only to incremental improvements
in precipitation 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 remarks.
###
Michigan Technological University
Thanks to Michigan Technological
University for this article.