Cloud Seeding And It's 3 Types Of Seeding

Cloud seeding with history and types of cloud seeding like dry ice, silver iodide and warm cloud.

Cloud Seeding And Their Types

Cloud seeding

Cloud usually consist of very small droplets of water which cannot fall appreciably below the cloud base without evaporating. In this condition Clouds are stable. Raindrops are about a million times heavier than cloud drops and rain develops only if the cloud droplets grow by some mechanism. Cloud seeding mechanism depends on the difference in vapour pressure over ice and supercooled water at the same temperature. If ice crystals form in the supercooled cloud, water evaporates of the dropleta and migrate to the ice crystals, which grows and begins to fall increasing their growth rate by coalescence with other droplets and eventually melting and falling as rain drops. Presence of ice nuclei determines whether ice crystals form in a supercooled cloud or not. Ice crystals form on naturally occurring nuclei at temperature of -10° to -30℃. It warmer temperature, either the efficient ice forming nuclei are often deficient in the atmosphere or the cloud may occur with tops supercooled but not to an extent which activates the eyes nuclei available. It should be possible to initiate precipitation by introducing ice crystal or by supplying artificial nuclei.

History of cloud seeding.

Vincent Schaefer (1906-1993) discovered the principle of cloud seeding in July 1946. Along with the associate Langmuir, he created a way of experimenting with supercooled clouds using a deep freeze unit of potential agents (salt, talcum powder, soils, dust and various chemical agents) stimulate ice crystal growth. The experiment was easily replicated and he explored the temperature gradient to established the -40℃ limit for liquid water. Within the month, Scheafer's colleague, atmospheric scientist Dr Bernard vonnegut is credited with discovering another method for "seeding" supercooled cloud water.

Dry eyes and silver iodide agents are effective in changing the physical chemistry of supercooled Clouds, thus useful in augmentation of winter snowfall over mountains and under certain conditions and lightning and hail suppression.
In Inμ, cloud seeding operation conducted during the year 2003 and 2004 through US based weather modification INC in the state of Maharashtra. Cloud seeding in 12 districts of Andhra Pradesh was initiated in 2008.

Dry ice seeding.

It is to change supercooled cloud droplets to Ice crystal in the laboratory and in the atmosphere by dropping pellets of dry ice into the cloud and causing large number of ice crystals to appear spontaneously. Dry ice is solid frozen carbon dioxide at the temperature below -80℃. An air crafts drops dry ice pellets of size 0.5 to 1.0 cm at the top of supercooled Clouds. They fall through the clouds and result in the sheet of ice crystals. This method is reliable for stimulating showers from Cumulus clouds provided there tops have temperature lower than -5℃ in the clouds lasts from more than 30 minutes.

Silver iodide seeding.

Minute crystals of silver iodide produced  in the form of smoke acts as efficient ice forming nuclei at temperatures below -5℃ to produce enormous number of nuclei. When this smoke is introduced into supercooled cloud, some ice crystals appear when temperature fall below -4℃, but its formation rapidly increases with decreasing temperature. Cloud seeding by silver iodide is done either from ground generators or form airborne generators. Substances other than silver iodide as artificial nuclei or lead iodide, cupric sulphate, cupric oxide, ammonium fluoride, iodide and Iodine. However, all these are not effective as silver iodide.

Warm cloud seeding.

It is a process in which nuclei of droplets grow to radii for several microns in coalescence between drops. Once drops have grown to about 40 μ, they are capable of colliding with and sweeping up smaller droplets in their path. Once the droplets reach the size of 60 Μ, coalescence dominates and condensation can be virtually ignored. Coalescence process is mainly responsible for growth of raindrops in warm cloud. In this case, seeding would be based on the assumption that the presence of comparatively large water droplets is necessary to initiate the coalescence process and some clouds do not precipitate or do so inefficiently due to absence of such large water droplets. This deficiency can be remedied by introducing in the cloud, water droplets or hygroscopic nuclei. Salt particles of 3μ becomes 50 Μ particle of brine in saturated air of cloud. Water droplets of 20 to 30 μ introduced in the cloud through aircraft at the rate of 30 gallons per minute appears to be effective for obtaining the rain.

Experiments have clearly established that cloud seeding with silver iodide or dry ice in the case of cold clouds in sodium chloride or minute water droplets in the case of warm clouds can stimulate rain from suitable clouds. However, practical experience as of now, may not suggest recommending it for adoption is an option for drought during crop season.