November, 1946 marked a turning point in the range of science and technology available to humans. Bernard Vonnegut, Ph.D., (writer Kurt Vonnegut's brother), produced the research that gave us, in a sense, the capability to control weather.
Vonnegut discovered that microscopic crystals of silver iodide (AgI) created a nucleus around which water vapor in clouds could form ice crystals.
Silver iodide was the ideal compound because the distance between the molecules in its crystal lattice structure is nearly equivalent to that of ice. Vonnegut then applied this research to create the compound that is used today for cloud seeding. He dissolved silver iodide with additional iodide in acetone and then sprayed this solution through a nozzle to create droplets before burning them.
Yury Luzhkov, mayor of Moscow, is determined to bring Dr. Vonnegut's discoveries to his city this winter: He has promised that no snow will fall. Luzhkov's office has planned to hire the Russian Air Force to seed clouds prior to their arrival to the city, which will cause them to release snow outside of the city. They will use cement powder, dry ice or silver iodide to prevent the fall of the largest snow storms from mid-November through mid-March.
The investment of six million dollars into this project, approximately half of what the city typically spends on snow removal, is expected to improve quality of life and traffic in the city this winter, as it will not be necessary for road crews to constantly be clearing the snow.
Cloud seeding, which may be accomplished with substances that have a temperature below 40 degrees Celsius (including solid carbon dioxide pellets or the liquid forms of carbon dioxide, propane and nitrogen), is the term for when substances are released into a cloud whose temperature is between -39 and five degrees Celsius, converting water vapor into ice crystals. This process, known as sublimation, happens as the ice crystals increase in size around a nucleus of silver iodide, at the expense of shrinking water droplets. Latent heat is released from the conversion of water vapor to ice. The heated air rises within the cloud, ultimately increasing the flow of vertical air, or convective growth, within the cloud. The taller a cloud is, the larger the raindrops or snowflakes that fall from it will be.
The amount of water in the cloud will also increase, as more moisture will be pulled into its base through convection. Factors such as the temperature and humidity below the cloud will cause snow to change to rain or even evaporate prior to melting.
Another scenario occurs when aptly-named "warm clouds" do not contain large amounts of cooled water. The temperature of water droplets is greater than zero degrees Celsius. Zero degrees Celsius is actually the temperature at which ice melts. Water will freeze between zero and -39 degrees Celsius, depending upon whatever contaminants happen to be present. Thus, "warm clouds" cannot be seeded with silver iodide, as the air temperature is too great. However, an airplane may be employed to release hygroscopic materials (those that attract or absorb moisture from the air) such as sodium chloride, in the form of dust particles with a diameter of approximately 25 micrometers, to convert water vapor into liquid water.
The idea of cloud seeding is not new to Moscow. Its $40 billion budget per year has been used in the past to ensure that no rain occurs on the parades that take place on Victory Day, in May, and City Day, in September.
Luzhkov remarked in an interview with Time magazine that his proposal makes "financial sense." Less snow will be present to require clearing in the city of Moscow, while its suburbs will receive more moisture, allowing for an improved harvest.
Cloud seeding materials tend to be applied to areas in amounts that are small in comparison to the geographical size of the areas, thus producing low concentrations of seeding materials (for example, silver iodide tends to be used in concentrations below 0.1 micrograms per liter, which is below the U.S. Public Health Service's enforced limit of 50 micrograms per liter). However, Moscow's citizens have understandably posed some concerns. Alla Kachan, ecology minister of Moscow, declared that the plan still needs to be further examined by environmental experts and discussed with area residents before it is approved.
