June 11, 2009

Raindrop study implies incorrect forecasts

U.S. and Mexican physicists have found smaller raindrops can fall faster than bigger ones, meaning some weather forecasters have overestimated rainfall.

Scientists from Michigan Technological University and the National University of Mexico said it's been conventional atmospheric science wisdom that 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.

But in the new research scientists discovered some smaller drops not only can fall faster than bigger ones, but they can fall faster than their terminal speed -- in other words, they can fall faster than drops of that size and weight are supposed to be able to fall.

The researchers -- Michigan Tech Professors Alexander Kostinski and Raymond Shaw with Guillermo Montero-Martinez and Fernando Garcia-Garcia in Mexico -- said the findings could significantly alter scientists' understanding of the physics of rain and improve the accuracy of weather measurements and forecasts.

The study is reported in the June 13 issue of the journal Geophysical Research Letters.