Weather modification has been used in an attempt to increase rainfall and snowfall, to suppress hail, to clear fogs, to modify tropical cyclones and to suppress lightning. The term "weather modification" is not normally used to describe inadvertent changes in our weather. For example, heat and aerosols released from major city centres affect the temperature and precipitation pattern around those cities. Weather modification also does not refer to climate change caused by industrial and natural sources of aerosols and gases such as carbon dioxide.
People have attempted to modify the weather and climate for thousands of years. For example, native people performed elaborate dances. In Napoleonic Europe, cannons were fired during cloudy weather. Unfortunately, it is unlikely that any such techniques had more than psychological value. Scientific weather modification began in 1946 when American scientists Vincent Schaefer and Bernard Vonnegut discovered, in independent studies, that supercooled cloud droplets could be converted to ice crystals by a cooling agent such as dry ice, or an artificial ice nucleus such as silver iodide.
Most weather modification projects attempt to change the rate at which clouds form precipitation through the injection of ice nucleating chemicals. These chemicals cause supercooled cloud droplets (those below 0°C) to freeze into ice crystals, and these ice crystals grow much more rapidly than the droplets. The ice crystals then begin to fall and sweep out more cloud droplets and ice crystals. Precipitation then falls in the form of snow or rain, depending on whether they fall into regions warmer than 0°C. This is the dominant precipitation formation mechanism in Canada, and the artificial ice nucleant merely accelerates the process.
Hail suppression techniques attempt to create many smaller hail embryos and thus increase the possibility the hail will melt before it hits the ground. For supercooled fog dispersal, ice crystals are generated that deplete the small droplets and thus increase visibility. Some projects attempt to totally freeze all the liquid in the cloud, thus quickly releasing the latent heat of freezing, which increases the buoyancy of the cloud, and results in larger and more active clouds. It is also possible to modify clouds or fogs that are entirely warmer than 0°C. This is done through the use of efficient condensation nuclei that create larger and fewer droplets in the cloud, which accelerates the precipitation formation process.
A few experiments have attempted to reduce lightning by using rockets to fire thin metallic wires into growing clouds to trigger and channel premature lightning strokes. In short, a variety of weather modification techniques have been used around the world.
In Canada weather modification began in 1948 with a federal government experiment that used dry ice dispersed into clouds to stimulate rainfall. Under appropriate conditions, rainfall did result. However, as with most weather modification projects, the question of what would have happened naturally could not be properly answered. A randomly selected control population of unseeded clouds was not available for comparison with the population of seeded clouds. Nonetheless, in spite of scientific uncertainties, the 1950s saw a blossoming of rainmaking activities on the prairies for agricultural purposes, and in eastern Canada for forestry and hydroelectric power. Silver iodide was the seeding agent, dispersed variously from ground-based and airborne generators. These operations were not designed as scientific experiments, and later analyses were inconclusive.
In 1959 one of the first of a series of international statistical rainmaking experiments was mounted by the federal government in northeastern Ontario and northwestern Québec using aircraft to seed clouds with silver iodide. The outcome of this 4-year experiment on large-scale storm systems was an overall small decrease in rainfall. The decrease was not statistically significant and could have been the result of chance. However, an operational rainmaking project in the Lac Saint-Jean area, Qué, was perceived by residents as having been very successful - so successful, in fact, that "Operation Umbrella" was mounted and mothers petitioned the Québec government for vitamins for their children because of lack of sunshine. In 1965 the Québec minister of natural resources ordered all rainmaking activities in the province to cease.
In general, rainmaking declined throughout Canada during the 1960s. The exception was Alberta, where the interest in weather modification remained high and operational and research projects to suppress hail and increase rainfall continued into the 1980s.
Weather modification projects usually have objectives that do not satisfy everyone. For example, farmers might want more rain while tourists would prefer more sunshine. In addition, although a target area is usually selected, the possibility exists the effects might be felt in nearby communities, provinces or countries. Consequently, some provinces and the federal government have legislation that enables them to monitor, and in some cases, licence, anyone attempting to modify the weather.
There have been very few demonstrated successes in weather modification around the world. Some Canadian advances have been the ability to monitor the change in cloud properties due to ice nucleants, and to numerically simulate with computers the possible changes in clouds. Elsewhere, certain techniques have been generally effective in clearing fogs, and a procedure to produce more snowfall from clouds forming over mountains also shows some promise. There have also been a few reported successful experiments to produce rainfall. There have been no hail suppression experiments that the scientific community has fully accepted.
Randomized tests are scientifically considered the best method for evaluating weather modification. Considering the natural variability of precipitation and the generally low values of expected effects (10-20%), randomized tests would generally need to be conducted for 5-10 years before statistically significant results are obtained. This makes weather modification research costly and difficult to perform. There are also many unknowns concerning cloud microphysical processes and the generation of cloud systems. Consequently, much more work remains to be done before the weather can be modified in a controlled manner.