Rain is so closely studied by meteorologists that it’s easy to imagine that it long ago gave up its secrets to science. Not so. Rain drops are not fully understood and perhaps their greatest secret is how they form.
Meteorologists know the general details, of course. As water vapour rises, it cools to form a fine mist of water droplets about 1 micrometre in diameter. These droplets grow in size until they are heavy enough to fall, triggering rain.
The size of the droplets in this process is critical—they must be at least 50 micrometres in diameter before they can fall. Meteorologists have observed this process in action and say it takes about 15 minutes.
But therein lies the problem: droplet growth should take hours according to current theories. Consequently, nobody knows how rains drops form so quickly, a problem known as the condensation-coalescence bottleneck.
Now Tov Elperin at Ben-Gurion University of the Negev in Israel and a few buddies say they’ve solved the conundrum in a paper submitted to the journal Atmospheric Chemistry and Physics. These guys say a new type of turbulence discovered earlier this year finally explains the accelerated growth of rain droplets.
First some background. In clouds, droplets form when water vapour condenses onto aerosol particles. These droplets are only a micrometre or so in size and tend to stay that way because the rate of condensation is balanced by the rate of evaporation.
The only way for droplets to grow is by coalescing. In other words, by colliding and fusing with other droplets.
But here’s the problem. Meteorologists say the density of droplets in clouds is too low for them to collide often enough to explain the observed rate of growth. Indeed, the density would need to be several orders of magnitude higher to explain the observations.
But Elperin and co now think they know what’s going on. Earlier this year, they discovered a new phenomenon at work in turbulent air that concentrates the small particles within it.
The new phenomenon is called tangling clustering instability and comes about because turbulence is made up of many small eddies or vortices. Any small particle trapped in an eddy gets flung out due to centripetal forces. So the particles become concentrated at the intersections between the eddies, raising their density in these regions by a factor of 100,000.
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