| Related Stories:
|
| Keywords: | |
|
Shower Power: What's Luck Got to Do with It?
For more information on this story contact:
If it rains on your parade next summer, enjoy the moment. You might be witnessing one of Nature's minor marvels.
Physics faculty members Alex Kostinski and Raymond Shaw have proposed an answer to a 50-year-old meteorology puzzle: How is warm rain initiated? They have published their findings in the February 2005 issue of the Bulletin of the American Meteorological Society in the paper “Fluctuations and Luck in Droplet Growth by Coalescence.”
Atmospheric scientists have a good understanding of how rain happens in cold conditions, when ice crystals form in clouds. But warm rain is another matter.
"Imagine it's 2 p.m. in Florida," says Kostinski. "You can just barely see a cloud form, and a half hour later it rains. It is surprisingly quick."
Here's why. Clouds are made up of droplets only a few microns in diameter. Raindrops are a millimeter or so across, that is, a million times bigger in terms of volume. Therefore, in order to form a raindrop, a million droplets must collide and coalesce. How can all this happen in 20 to 30 minutes? The odds that any single droplet will collide with enough other droplets to form a raindrop so quickly are about one in a million.
Apparently, however, that's all it takes. If a few lucky droplets prevail against all odds and bang into five or six others, they can really gather up steam, collide with lots of other droplets, and voila! raindrops start falling on your head.
So why do only some of those ephemeral summer clouds form rain? First, there's the presence of turbulence. If the droplets are getting blown around, they are much more likely to crash into each other and form raindrops.
Secondly, not all droplets are created equal. Kostinski and Shaw note that some are bigger than others because they have condensed around larger particles of dust.
And lastly, sun showers depend on a random sprinkling of chance. "To understand warm rain, you don't have to explain how the average drop grows in a cloud," says Kostinski. "Just the fastest, 'luckiest' ones." |
|
