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Rain isn't just a soothing sound. It also helps calm the winds. How? Friction.
That's according to a new study published in Science on February 24.
Winds and even the atmospheric jet streams caused by the rotation of the planet dissipate much of their energy via friction already. Air molecules hurled hither, thither and yon rub together and warm up. But rain and other forms of precipitation rub against air molecules too, and produce a tiny bit of friction. That resistance slows each drop’s fall and also slows the winds themselves.
Raindrop drag hadn't been measured before, but, using satellites, scientists found that this friction averages 1.8 watts per square meter in the tropics. And that adds up on a global scale. Thanks to climate change even heavier downpours that originate higher in the sky are expected in the future. That means even more friction and, potentially, less of the wind that mixes the atmosphere.
Of course, the extra heat trapped by human greenhouse gas emissions is likely to play a bigger role than raindrop friction in any atmospheric changes. But it's going to be a more humid, wetter world. Better get a raincoat.
—David Biello
[The above text is a transcript of this podcast.]
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1 Comments
Add CommentRaindrops are interesting in another way. Clouds are visible because they are formed out of tiny raindrops. Raindrops are larger than droplets in clouds, and so they fall out of clouds. Actually, droplets in clouds seem to float because water vapor is lighter than air. Cloud droplets do fall, but as the tiny droplets fall they evaporate because air is warmer at lower altitude.
Reply | Report Abuse | Link to thisWater vapor is lighter than air, and so water vapor rises in air until it forms clouds. Rising water vapor affects weather since rising vapor, as it condenses, warms cold air, which hampers cloud formation thus allowing sunlight to reach Earth's surface.
Tiny raindrops in clouds add hysteresis to weather, so that rain or dry weather occurs. Falling raindrops tend to keep weather wet by slowing wind and thus slowing evaporation.
Raindrops affect weather, but CO2 does not. CO2 already blocks as much infrared as it can, and so more CO2 cannot block more infrared (see http://carbon-sense.com/wp-content/uploads/2008/05/hertzberg.pdf).