Super-Terminal Speeds

Interesting! And plausible too!:

Many raindrops travel at "super-terminal" velocities, faster than was thought possible. As a result, meteorologists may be miscalculating how much it rains.

Previously, it was assumed that all raindrops fall at terminal velocity, a constant maximum speed that is determined by the interplay of gravity and drag. The velocity for individual drops is considered to be largely controlled by their size: larger drops fall faster than smaller drops, due to their greater mass.

Fernando García-García of the National Autonomous University of Mexico and colleagues measured the shadows of natural raindrops passing through a ray of infrared light. They found that up to half exceed their terminal velocity. Some travel as much as 10 times faster, for their size.

"Others had detected this before, but everybody disregarded it, blaming it on an error," says Garcia-Garcia.

The team suspects that the super-terminal drops may be fragments of larger drops broken apart as they fall. "If a large drop breaks into several fragments, each drop will have the speed of the large drop, at least temporarily, until the smaller drops slow to their new terminal velocity," García-García says.

As a result, meteorologists may be overestimating total rainfall by up to 20 per cent, say the team. Weather forecasters use total rainfall figures to predict floods, and climatologists use the estimates to gauge how rain patterns are changing with climate change.

...Ana Barros of Duke University in Chapel Hill, North Carolina, says meteorologists' rainfall estimates are unlikely to be too far wrong, pointing out that the super-terminal drops would soon slow down after fragmenting from larger drops.

Journal reference: Geophysical Research Letters, DOI: 10.1029/2008GL037111 (in press)