by: William R. Cotton
Colorado State University
Department of Atmospheric Science
Fort Collins, CO 80523
In this paper I will focus only on three methods of seeding clouds. The first two are related to supercooled clouds and are called the "static mode'' of cloud seeding and the "dynamic mode'' of cloud seeding. The third method is the modification of warm clouds by hygroscopic seeding. (image at right from Utah.gov, click image for detail)
The main objective of the "static mode'' of cloud seeding is to increase the efficiency of precipitation formation by introducing an "optimum'' concentration of ice crystals in supercooled clouds by cloud seeding.
It was originally thought that clouds were deficient in ice nuclei and therefore additions of modest concentrations of ice nuclei should result in a more efficient precipitation-producing cloud system.
All that was needed was to introduce seeding material from the ground or at the base of clouds which would then enhance ice crystal concentrations and thereby increase rainfall.
While the fundamental concept of the 'static mode' of cloud seeding is that precipitation can be increased in clouds by enhancing their precipitation efficiency, alterations in the dynamics or air motion in clouds due to latent heat release of growing ice particles, redistribution of condensed water, and evaporation of precipitation is also inevitable.
Alterations in the dynamics of clouds, however, is not the primary aim of the strategy. By contrast, the focus of the 'dynamic mode' of cloud seeding is to enhance the vertical air currents in clouds and thereby vertically process more water through the clouds resulting in increased precipitation.
One method of seeding clouds to enhance precipitation is to introduce hygroscopic particles (salts) which readily take on water by vapor deposition in a supersaturated cloudy environment.
The conventional approach is to produce ground salt particles in the size-range of 5-100 [particles] , and release these particles into the base of clouds.
These particles grow by vapor deposition and readily reach sizes of 25 to 30 [particles] in diameter or greater. They are then large enough to serve as "coalescence'' embryos and initiate or participate in rain formation by collision and coalescence.(image below example of Hygroscopic Seeding, from HyrdroTasmania)