The scientific base of weather modification was laid down in the 1940’s by American scientists, participating in a group lead by the Nobel prize winner Irving Langmuir.
In 1946, Vincent Schaefer discovered a method of artificial formation of ice crystals in supercooled clouds in the course of experiments in a cloud chamber using solid carbon dioxide.
On November 13, 1946, Schaefer carried out his first experience in natural conditions, modifying small cumuli clouds by seeding them with dry ice from an airplane.
In November 1946, Bernard Vonnegut (the brother of the novelist Kurt Vonnegut) found out that silver iodide (AgI) had a similar structure to ice and had an iceforming property. This discovery provides a possibility for wide cloud system seeding.
In order to prevent hail damages, it is necessary to transform the dangerous convective clouds so as not to allow the formation of large hailstones. Usually, the number of ice crystals in the cloud is small and, upon the existence of appropriate conditions, they grow rapidly to hailstones with large sizes.
The most popular hail suppression concepts are:
- Beneficial competition
Seeding increases significantly the ice embryos concentration so that the artificial and natural ice particles compete with each other for available liquid water. The supercooled water redistributes between all ice embryos and thus resulting hailstones are small. Falling to the ground, they melt to rain or sleet.
- Early rainout
Seeding accelerates precipitation development, resulting in the “rainout”of still small hydrometeors from convective turrets that have not yet developed updrafts strong enough to support the growth of hail. Thus, the cloud supercooled water content (from which hail might otherwise grow) is reduced.
- Trajectory lowering
Seeding accelerates hail embryos growth at lower level in the cloud, where liquid water content is smaller and updrafts are weaker. Then the hail falls out of the cloud both in smaller sizes and earlier than if that would have taken place naturally.