Weather modification is the effort of man to change naturally occurring weather, for the benefit of someone. The best-known kind of weather modification is cloud seeding, with the goal of producing rain or snow, suppressing hail , or weakening hurricanes.
It is a common misconception that pure water freezes at a temperature of zero celsius (32 degrees Fahrenheit). Zero celsius is actually the temperature at which ice melts. Water freezes at a temperature between 0 and -39 celsius, depending on the type of nuclei (i.e., contaminants) present. Liquid water with a temperature of less than 0 celsius is called "supercooled water".
In November 1946, Dr. Bernard Vonnegut discovered that microscopic crystals of silver iodide (AgI) nucleate water vapor to form ice crystals. Vonnegut choose AgI crystals because there is nearly the same distance between molecules in the crystal lattice for both ice and AgI, which makes AgI the optimum material to nucleate ice. (Vonnegut, 1947) Vonnegut's discovery is a classic example of doing the right thing for the right reason at the very beginning of new technology, as a result of scientific knowledge. (Usually, progress is made by a series of small improvements on past practices, as a result of bumbling and guesswork. In contrast to the usual way, Vonnegut used his scientific knowledge to make a giant leap that has persisted as the state-of-the-art for more than fifty years.)
Vonnegut not only discovered the ice-nucleating properties of AgI, but he also invented a practical way of generating tiny AgI particles to serve as nuclei for ice crystals. Vonnegut dissolved a mixture of AgI and another iodide in acetone, sprayed the solution through a nozzle to make droplets, then burned the droplets. (Vonnegut, 1949; Vonnegut & Maynard, 1952) In this way, one gram of AgI can produce 1016 nuclei for ice crystals. More than fifty years later, Vonnegut's method continues to be the common way to seed clouds.
Release of AgI into an existing supercooled cloud (i.e., air temperature between -39 and -5 celsius) can convert water vapor to ice crystals, which is called sublimation. The ice crystals nucleated by the AgI will grow and local water droplets will shrink. The latent heat released by converting water vapor (or liquid water) to ice will increase vertical air motion inside the cloud and aid the convective growth of the cloud. Raindrops or snowflakes will grow larger by falling through a taller cloud. Also, moist air from evaporated moisture in the soil will be sucked into the base of the cloud by convection (i.e., updraft), thus increasing the total amount of water in the cloud. Perhaps 30 minutes after the AgI release, snow may fall below the cloud. Depending on the temperature and humidity below the cloud, the snow may change to rain, or even evaporate, before reaching the ground.
AgI is the most common ice nucleus used in cloud seeding, but it is not the only material used. Substances with temperatures less than -40 celsius (e.g., solid CO2 ["dry ice"] pellets, liquid CO2, liquid propane, liquid nitrogen, etc.) can be dropped from airplanes into the tops of clouds, to induce formation of ice crystals.
Seeding warm clouds:
Clouds that do not contain appreciable amounts of supercooled water are known as "warm clouds". In warm clouds, most of the liquid water droplets will have temperatures greater than zero celsius. Seeding warm clouds with AgI or dry ice makes no sense, because the air temperature is too high. Seeding clouds with either a water spray or hygroscopic materials requires dumping a large mass (e.g., 1000 kg) of seeding materials from an airplane, in contrast to the much smaller amounts of AgI needed to seed a different kind of cloud.
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by Paul Chehade (few years ago!)