Skip to comments.Warp-Speed Raindrops
Posted on 06/15/2009 6:28:28 AM PDT by JoeProBono
It's a rain race out there. In the meteorological equivalent of breaking the light-speed barrier, new research shows that the smaller droplets in a rainstorm often surpass what appears to be the speed limit for rain. The findings should help scientists devise models that could lead to more accurate weather forecasts.
Common sense dictates that larger raindrops should fall to the ground faster than smaller ones because they weigh more and can better overcome wind resistance. But anecdotal meteorology data have shown that when drops land, smaller ones are sometimes going just as fast as the biggest ones. That irregularity had puzzled scientists for many years; they usually attributed it to instrument problems. But now a team of physicists from Michigan Technological University (MTU) in Houghton and the National University of Mexico in Mexico City has found evidence that the phenomenon is real.
Over several years, the team clocked about 64,000 raindrops falling in Mexico City. The researchers measured their sizes and velocities only in extremely calm conditions, so the wind that often accompanies rain could not skew the data. They found that some drops plummeted faster than the so-called terminal velocity for their size--the speed, based on a well-established scale, at which air resistance counteracts the accelerative force of gravity.
Like the speed of light, the terminal velocity should be an absolute limit. But in a paper in press at Geophysical Research Letters, the team reports many observations of so-called superterminal drops, which form when larger drops collide and break up into bunches of small drops. Those smaller drops can then travel for a time as fast as the larger drops. For example, drops with a diameter of 100 micrometers are supposed to be limited to a terminal velocity of about 30 centimeters per second. But the researchers observed such drops hitting the ground at 3 to 4 meters per second.
"What surprised us was not so much seeing the superterminal drops," says physicist and co-author Raymond Shaw of MTU, "but seeing the deeper, compelling patterns." He explains that as rain falls harder, the fraction of superterminal, or speeding, small drops increases. At the same time, the proportion of the bigger drops decreases. That result, Shaw says, is "consistent with the notion that large drops break up to produce a swarm of speeding satellite droplets."
Shaw says there's a practical side to the research. "Weather forecasting models depend on simplified theories of how raindrops grow, [so] the more we understand about the interactions between drops, ... the more we can improve our ability to predict whether it will rain on tomorrow's picnic."
Yep, high tech tools, great computer models, and the forecasters are worse than ever. I can remember, back in the day, they could normally get tomorrow right. Now, they have trouble getting this afternoon right.
I thought Galileo debunked this kind of reasoning with an experiment involving a large sphere and a small sphere dropped simultaneously from a tower.
With something as light as a raindrop, the thickness of the atmosphere comes into play. A 100lb cannonball would fall faster (eventually) than a 100lb human, because they have different terminal velocities, since humans have higher wind resistance than cannonballs.
As you increase mass, inertia increases proportional to force, so the same gravitational constant applied to objects of two different mass will result in the same acceleration... ALL OTHER THINGS BEING EQUAL.
However, as an object accelerates, the amount of friction it creates by passing through the air increases. So the force (which causes further acceleration) stays constant, but resistance to that force (which opposes further acceleration) increases with speed. Eventually, resistance equals the gravitational force, and you reach a constant speed, where acceleteration = 0. This is called terminal velocity.
Although in Galileo’s case, this difference was too small too measure, so the experiment worked; he chose two objects with similar amounts of resistance. Had he compared the rates at which a feather falls to that of a ball, you can surmise from your own experience that wouldn’t have worked.
In Galileo’s case, the two balls each experienced small enough levels of resistance that the resistance was insignificant. As the fall for a longer time, and the difference in size increases, the resistance becomes more significant, to the point where tiny rain droplets have significantly slower terminal velocities that huge rain droplets.
>> So the force (which causes further acceleration) stays constant, <<
Well, of course gravitational force increases as you approach the Earth, but that increase is proportionally small for our purposes.
How's that common sense? The acceleration due to gravity is the same for all objects regardless of their weight and mass. Anyone who passed Jr. high physical science should know this. The terminal velocity of raindrops would necessarily be higher for smaller drops due to the low wind resistance.
of course, the difference is proportional to the shape and density of the human in question.
“How’s that common sense? “
You’re chewing out the wrong freeper.
Sorry FRiend, that wasn't aimed at you, but rather at the author who is clearly in over his head regarding the physics.
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