Weather modification through cloud seeding has been happening since the 1940s and continues today in 24 countries and ten U.S. states (Dick 2001). Although most government funding for research ceased in the late 1970s because of inconclusive results (Dick 2001) cloud seeding in the form of hail suppression, rain enhancement and fog dispersion, still continues (Weather Modification Inc). In the present day, advances in chemical technology and the resurgence of research on the subject may make weather modification much more prevalent in the future. Controlling the weather could, in some peoples eyes, prove to be one of mans greatest achievements or it could bring about dire consequences from trying to manipulate natural processes without fully understanding the results.
The concept of cloud seeding stems from the fact that when a cloud cools to less then freezing the water droplets do not immediately freeze but become super cooled (Breuer 1976). If temperatures become cold enough (-20 to -40 Celsius) water droplets will begin to freeze onto tiny soil or dust particles inside the storm- these are called ice nuclei- and then form snowflakes which will be heavy enough to fall and turn to rain as they exit the cloud (WMI). If temperatures do not cool to the point where ice can form, then the droplets will most likely mix with outside air and evaporate (Breuer 1976). Now, if an agent such as dry ice, can be induced into the cloud it will serve as ice nuclei and freeze the water droplets, which will then fall as rain (Breuer 1976). This is exactly what Vincent Schaefer demonstrated in 1946 when he added dry ice to a cloud chamber with which he was experimenting (Dick 2001). Soon after, Bernard Vonnegut found that, by using silver iodide instead of dry ice, water droplets could be induced to freeze at temperatures as high as -4 degrees Celsius (Breuer 1976). The reason for this is that silver iodide's structure is very similar to ice (Breuer 1976).