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An obvious hole in a stratus deck due to cloud seeding with aircraft, using dry ice as a seeding agent. This is an example of cold cloud seeding, where supercooled cloud droplets are converted into ice crystals, which then precipitate out of the cloud deck. (USAF photo; boxed caption in the lower right reads "Effects of seeding Altostratus Clouds over Green Bay, Labrador: 45 minutes after seeding with dry ice". Photo and boxed caption obtained from Sewell, W.R.D., et. al., 1973: Modifying the Weather; Western Geographical Series, Vol. 9, University of Victoria, Victoria, British Columbia, Canada).
How is the cloud seeding accomplished?
Cloud seeding materials are released via ground-based and/or airborne systems (see pictures). Determination of the best suited method or combination of methods for a given project is based upon an assessment of a variety of factors. The seeding materials are applied to the clouds (sometimes targeted very carefully into very specific portions of clouds) so that the material has adequate time to affect the precipitation process, so the effect will be focused over the intended geographic area.
A leading hypothesis holds that the hole-punch cloud is caused by falling ice-crystals. The ice-crystals could originate in a higher cloud or be facilitated by a passing airplane exhaust. If the air has just the right temperature and moisture content, the falling crystals will absorb water from the air and grow. For this to happen, the water must be so cold that all it needs is a surface to freeze on. The moisture lost from the air increases the evaporation rate from the cloud water droplets so they dissipate to form the hole. The now heavier ice crystals continue to fall and form the more tenuous wispy cloud-like virga seen inside and just below the hole. Water and ice from the virga evaporates before they reach the ground.