Cloud seeding

Mark Twain once said that everyone talks about the weather , but no one ever does anything about it. Although he may have been correct in his day, since the 1940s, researchers have been at least partially successful in modifying one aspect of the weather—precipitation.

After about three years of investigative work at the General Electric Research Laboratory in Schenectady, New York, researchers Irving Langmuir and his assistant, Vincent Joseph Schaefer, created the first human-made rainfall. Their work had originated as war-influenced research on airplane wing icing. On November 13, 1946, Schaefer sprinkled several pounds of dry ice (frozen carbon dioxide ) from an airplane into a supercooled cloud, a cloud in which the water droplets remain liquid in sub-zero temperatures. He then flew under the cloud to experience a self-induced snowfall. The snow changed to rain by the time it reached Langmuir, who was observing the experiment on the ground.

Langmuir and Schaefer selected dry ice as cloud "seed" for its quick cooling ability. As the dry ice travels through the cloud, the water vapor behind it condenses into rain-producing crystals . As the crystals gain weight, they begin to fall and grow larger as they collide with other droplets.

Another General Electric (GE) scientist who had worked with Langmuir and Schaefer, Bernard Vonnegut, developed a different cloud-seeding strategy. The formation of water droplets requires microscopic nuclei. Under natural conditions, these nuclei can consist of dust, smoke, or sea salt particles. Instead of using dry ice as a catalyst, Vonnegut decided to use substitute nuclei around which the water droplets in the cloud could condense. He chose silver iodide as this substitute because the shape of its crystals resembled the shape of the ice crystals he was attempting to create.

The silver iodide was not only successful, it had practical advantages over dry ice. It could be distributed from the ground through the use of cannons, smoke generators, and natural cumulonimbus cloud updrafts. Also, it could be stored indefinitely at room temperature .

There is general disagreement over the success and practicality of cloud seeding. Opponents of cloud seeding contend that there is no real proof that the precipitation experienced by the seeders is actually of their own making. Proponents, on the other hand, declare that the effect of seeding may be more than local.

Over the years, cloud seeding has become an accepted part of the strategy to combat drought . It may indeed bring crop-saving relief to a dry field or may help reinforce subsurface water tables. However, the practice has not begun to eliminate deserts or devastating droughts, for researchers have yet to reproduce the general ground-soaking effects of a well-organized natural storm system so necessary for agriculture and replenishment of water reserves. And today there are environmental concerns over any activity that threatens to change or destroy a bio-community such as the desert .

As researchers collect and analyze more information about the weather, other attempts to modify it are bound to be developed.

See also Air masses and fronts; Weather forecasting; Weather forecasting methods

cloud seeding Process of introducing nuclei, e.g. silver-iodide crystals or solid carbon dioxide (dry ice), into clouds composed of supercooled water droplets, in an attempt to induce precipitation. Dry ice introduced (at −80 °C) from the air into cloud lowers the air temperature so that (particularly at temperatures below −40 °C) some of the supercooled water droplets are converted into ice crystals which then grow by collisions with further droplets. Silver iodide (which has a crystal structure similar to that of ice), introduced from the air or ground, is the substance most commonly used in seeding: its crystals act as ice nuclei. Other substances, e.g. common salt or fine water droplets, may also be used to encourage coalescence. Natural seeding may be significant in cases where ice crystals from a high ‘releaser’ cloud (e.g. altostratus or cirrostratus) fall into a supercooled water ‘spender’ cloud (e.g. nimbostratus) and encourage ice-crystal growth.

cloud seeding The introduction of nuclei, e.g. silver-iodide crystals or solid carbon dioxide (dry ice), into clouds composed of supercooled water droplets, in an attempt to induce precipitation. Dry ice introduced (at −80°C) from the air into cloud lowers the air temperature so that (particularly at temperatures below −40°C) some of the supercooled water droplets are converted into ice crystals, which then grow by collisions with further droplets. Silver iodide (which has a crystal structure similar to that of ice), introduced from the air or ground, is the substance most commonly used in seeding: its crystals act as ice nuclei. Other substances (e.g. common salt or fine water droplets) may also be used to encourage coalescence. Natural seeding may be significant in cases where ice crystals from a high ‘releaser’ cloud (e.g. altostratus or cirrostratus) fall into a supercooled water ‘spender’ cloud (e.g. nimbostratus) and encourage ice-crystal growth.