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Biogenic Ice Nuclei
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12539 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
7 / 1987 |
3,064 Words |
| Author
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Joann T. Dennett
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Single-celled organisms may have been at the heart of the latest thundershower, according one theory now gaining support. At the same time, down on the ground, the dispersal of single-celled organisms in tests to inhibit frost damage on food crops in California is the subject of heated legal battles.
The microbes at issue are Pseudomonas syringae, which secrete a protein crystal that enhances the formation of ice crystals from super cold water. This ice-nucleating characteristic is central to a singular example of the interconnectedness of life on our planet that is now being uncovered. Recent research has linked the microscopic P. syringae with the massive thunderstorm clouds that can tower 40,000 feet or more into the atmosphere.
Originally at home on a plant leaf, these microbes can be lofted skyward together with many other aerosol particles, where they may trigger the transformation of cloud droplets into the ice crystals that are essential to the formation of rain in many types of clouds. Once an ice crystal is nucleated in a supercooled cloud, it can then grow rapidly as the vapor in the cloud condenses on it.
Although it is commonly thought that water freezes at 0C, in the absence of suitable particles to trigger crystallization, water must be cooled to minus 40C before it will freeze. Often the temperature inside clouds drops well below 0oC before any freezing occurs - a phenomenon known as supercooling.
Research indicates that rain frequently falls from clouds whose temperature is no lower than minus 15C. For ice to form at such a "warm" temperature, there must be ice nuclei of some substance other than water in the cloud. Although no tests have confirmed which ice nuclei are active in rain formation, circumstantial evidence suggests that the biogenic ice nuclei produced by P. syringae may be central.
For a raindrop to form without starting as an ice crystal, millions of cloud droplets must coalesce by successive collisions - a slow and often inefficient process. Therefore, biogenic and other ice nuclei, which start ice formation at relatively warm temperatures, clearly provide a wide margin of advantage for precipitation formation.
Contrary to what some may think, clouds are not composed of pure water. Along with small particles of dirt and clay, a variety of microbial life inhabits humid clouds. Spores, seeds, fungi,
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