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Scientists get to the bottom of rain


Dutch scientists believe they have discovered how water vapor in clouds forms rain.
Dutch scientists believe they have discovered how water vapor in clouds forms rain.  

November 16, 1999
Web posted at: 9:57 a.m. EST (1457 GMT)

Thanks to a Dutch research team, we now have a better handle on how rain is produced in clouds, which should improve weather predictions, our understanding of the chemical reactions in the atmosphere and the absorption and reflection of solar radiation.

Clouds contain disproportionately large quantities of large and small water droplets, something meteorologists have been unable to explain until recently.

Now Dutch researchers at Delft University of Technology say spiral patterns, or vortices, a few centimeters in size force the droplets to the edge of the cloud, eventually producing rain.

The researchers calculated how hundreds of thousands of water droplets contained in about one liter (about one quart) of cloud move and grow. During this process, they hypothesized that tubular-shaped vortices, whirling masses of water, a few centimeters in size are formed and that these force the water droplets outward by centrifugal force, so that they congregate at the edge of the cloud.

For rain to condense and fall to the ground as precipitation, approximately one in every million droplets needs to acquire a diameter greater than 20 micrometers, which it does by colliding with other droplets. A chain reaction then takes place. Previous meteorological calculations had not considered the effect of small-scale areas of turbulence. The turbulence causes small droplets to collide more frequently than expected, the scientists speculate. It takes about 30 minutes for a cloud to become "ripe" enough to release rain.

A night-time thunderstorm illuminates storm clouds and bands of rain.  

Almost no droplets are present at the center of each area of turbulence, so that the air there remains extremely supersaturated, the scientists discovered. This may mean that air higher than about one hundred meters (328 feet) above the bottom of a cloud also becomes so supersaturated with water vapor that droplets are created. Until this study, meteorologists considered this to be impossible.

A cloud is a rising bubble of moist air in which the temperature drops as a result of expansion. This causes the air in the cloud to become supersaturated with water. The water vapor then condenses on particles, such as aerosols, with a radius of less than one micrometer. The amount of small droplets that develop depends on the level of supersaturation of the air.

The research team used the Netherlands Organization for Scientific Research's CRAY C90 supercomputer at the Academic Computation Center in Amsterdam. The Netherlands Organization for Scientific Research Council financed the project for Geosphere and Biosphere Sciences.

Copyright 1999, Environmental News Network, All Rights Reserved

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Netherlands Organization for Scientific Research
Clouds and Precipitation: online meteorology guide
National Weather Service.
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