Maybe it's raining less than we thought

Jun 11, 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.

Source: Michigan Technological University (news : web)

Explore further: NASA sees Hurricane Edouard far from US, but creating rough surf

add to favorites email to friend print save as pdf

Related Stories

There is a dark side to the humble raindrop

Jan 19, 2007

A single drop is harmless, but when billions of raindrops fall from a cloudburst onto bare soil they strike like billions of tiny hammers, dislodging tons of soil per acre which is carried away by surface runoff.

Building a Better Virtual Raindrop

Jun 26, 2005

A new way of mathematically modeling the formation of rain drops in clouds may improve our understanding of Earth’s climate, cloud formation and movement, and the effect that small airborne particles have ...

Study may produce better weather forecasts

Aug 11, 2005

Accurately forecasting rain reportedly will become easier thanks to a study of clouds conducted by the University of Leeds and University College London.

Rain Power: Harvesting Energy from the Sky

Jan 22, 2008

Researchers who study energy harvesting see energy all around us – we just need to find a way to capture that energy. One of the latest energy harvesting techniques is converting the mechanical energy from ...

Recommended for you

NASA image: Agricultural fires in the Ukraine

41 minutes ago

Numerous fires (marked with red dots) are burning in Eastern Europe, likely as a result of regional agricultural practices. The body of water at the lower left of this true-color Moderate Resolution Imaging ...

NASA marks Polo for a hurricane

1 hour ago

Hurricane Polo still appears rounded in imagery from NOAA's GOES-West satellite, but forecasters at the National Hurricane Center expect that to change.

NASA sees Hurricane Edouard enter cooler waters

1 hour ago

NASA's Tropical Rainfall Measuring Mission or TRMM satellite and Aqua satellite gathered data on Hurricane Edouard's rainfall, clouds and waning power is it continued moving northward in the Atlantic into ...

First eyewitness accounts of mystery volcanic eruption

4 hours ago

New light has been shed on one of the biggest volcanic eruptions in the last 500 years—the so-called 'Unknown eruption'—thanks to an unusual collaboration between a historian and a team of earth scientists at the University ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

mongander
not rated yet Jun 12, 2009
Scientists have determined that London mist contains more droplets per inch of rain than does a tropical monsoon.