Why Rain? Why Snow?

After lo, these many years, most meteorologists remain baffled by a couple of apparently simple questions: "Why does it rain?" and "Why does it snow?"

This much is known: most clouds are made of water droplets so tiny that they behave something like the particles in smoke. They cannot gather into larger drops unless an outside "something" makes them do it. In temperate climates this something is generally the low temperature of high altitudes, which turns some of the droplets to small ice crystals. Since ice has an attraction for water, these droplets grow at the expense of the cloud's smoke-sized water droplets. Eventually the cloud turns to snowflakes that are big and heavy enough to fall through the air and hit the earth as snow or rain. All this can sometimes be made to happen by releasing materials (such as silver iodide smoke) that encourage ice crystals to form in a moderately cold cloud. But this is an artificial process; in nature some clouds give snow and rain while others just as cold do not.

Cool Droplets. In Scientific American, Cloud Physicist B. J. Mason of London's Imperial College of Science and Technology tells of experiments made to determine why some clouds give rain while others float high in the air until they evaporate. When he carefully cooled small droplets of very pure water, they did not turn to ice until the temperature fell below -- 42DEG F. This proved, as had been suspected, that ice crystals seldom, if ever, form in moderately cold clouds unless some solid nucleus is present to start the process.

Mason's next step was to cool droplets containing microscopic nuclei made of substances that are common in powder-fine dust blown up from the earth's surface. A few kinds proved almost as effective as silver iodide smoke, but most required very low temperatures before they could turn cold clouds into snow.

Trained Dust. In further experiments, Mason showed that some kinds of common natural dust can be "trained" to collect ice. Particles of kaolinite (common in clays) do not act as ice-forming nuclei above 16DEG F., which is colder than the tops of many clouds. But when kaolinite particles have once had ice crystals on them, and when this ice has evaporated, they are able to form fresh crystals in clouds no colder than 25DEG F.

Mason suspects that kaolinite and other "trainable" particles are carried up to 35,000 ft., where the temperature often falls to --60DEG F. There they gather a little ice, forming thin, veil-like cirrus clouds. When they fall through dry air, most of the ice evaporates, but tiny bits remain trapped in crevices. When these ice-seeded particles get mixed with a moderately cold cloud, they make it yield snow or rain. Mason argues that much of the earth's precipitation is wrung out of clouds by just such "trainable" earth-dust particles. Kaolinite and other kinds of clay are extremely cheap, so it may be possible to make sure that the air over thirsty countries always has plenty of just such particles--always ready, willing and able to precipitate clouds.

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