Cloud seeding has been in existence for many years. In the late 1940s, Nobel Laureate Irving Langmuir and atmospheric scientist Vincent Schaeffer conducted a classic experiment that demonstrated that intentional human modification of the weather was possible. Their experiments found that home freezers containing super-cooled droplets could be changed to ice crystals by dropping specks of dry ice or frozen carbon dioxide into the cloud chamber. Once passing through warmer  atmospheric layers, the transfer to a liquid form would occur. Later experiments found that silver iodide, and sea salt aided in precipitation formation.  These procedures were originally implemented in drought-stricken agricultural areas. Proceedings of the National Academy of Sciences noted that another unassuming means of weather modifications occur regularly. Turboprop and jet aircraft chill the air in mid-level clouds, causing bands of snow and rain to develop and fall. Holes that are left behind in the clouds are called hole-punch or canal clouds and scientists believe this is evidence of inadvertent cloud seeding.  The super-chilled droplets that drop to -5 degrees are the key ingredient for hole development.  Once the air is chilled behind the aircraft propellers and over jet wings, the droplets freeze and disperse to Earth.  The passage of the aircraft produces as much precipitation as with cloud seeding materials.