Cloud Seeding.

Chemical trails are long lasting and do not dissipate in minutes like conventional condensation trails.
Chemical trails are long lasting and do not dissipate in minutes like conventional condensation trails.

Throughout history and probably prehistory man has sought to modify weather by a variety of means. Many primitive tribes have employed witch doctors or medicine men to bring clouds and rainfall during periods of drought and to drive away rain clouds during flooding episodes. Numerous examples exist where modern man has shot cannons, fired rockets, rung bells, etc. in attempts to modify the weather (Changnon and Ivens, 1981).

It was Schaefer’s (1948a) discovery in 1946 that the introduction of dry ice into a freezer containing cloud droplets cooled well below 0°C (what we call supercooled droplets) resulted in the formation of ice crystals, that launched us into the modern age of the science of weather modification. Working for the General Electric Research Laboratory under the direction of Irving Langmuir on a project investigating ways to combat aircraft icing, Schaefer learned to form a supercooled cloud by blowing moist air into a home freezer unit lined with black velvet. He noted that at temperatures as cold as –23°C, ice crystals failed to form in the cloud. Introducing a variety of substances in the cloud failed to convert the cloud to ice crystals. It was only after a piece of dry ice was lowered into the cloud that thousands of twinkling ice crystals could be seen in the light beam passing through the chamber.

Planned and Inadvertent Weather Modification/Weather Modification Association


Results of Cloud Seeding Operations for Precipitation Enhancement in Iran during 1999-2007

Morteza Khalili Sr., National Cloud Seeding Research Center, Yazd, Iran; and M. Seidhassani, F. Golkar, and V. Khatibi


Cold–cloud seeding operations to enhance precipitation over the Central part of Iran were carried out by government of Iran with cooperation of Central Aerological Observatory (CAO) during 1999-2001 & 2006-2007. The project operated to increase precipitation over the territory of Yazd province and adjacent regions using an AN-30 (1999-2001) seeding-research aircraft & two AN-26

(2006-07) seeding-research aircrafts. The operational area increased from approximately 125,000 km2 (1999) to 500,000 km2 (2006-07). The aircrafts was equipped with: 1) a measuring and data acquisition system enabling the processing, recording and real-time display of measured flight parameters atmospheric and cloud characteristics and 2) means of cloud seeding by ejecting PV-26 cartridges with AgI pyrotechnic flares and airborne liquid Nitrogen generators of fine- dispersion ice particles. For the effective organization and fulfillment of the operations the first automated weather radar system ACSOPRI-E was installed on the project territory in 1999, the second radar in 2001 and the third radar in 2007.

In order to achieve high seeding effectiveness over the target territory the operation were conducted in essentially all situations favorable for cloud seeding, including nighttime. This mode of operation hampers statistical evaluation of the result, since in the case it is impossible to select a fixed control area within the project site, which would not be affected by seeding. In this connection, a modified technique of historical regression, or ” Floating Control Method” was used for statistical evaluation.

Statistical evaluation of the effectiveness of regular cold-cloud seeding operation, carried out over the project territory in the Central part of Iran during the period of operation, shows that from 0.7 to 1.9 km3 of additional water was obtained about 22-40% of the natural seasonal precipitation annual.

Session 4, Updates on Research and Operation Programs: Winter Precipitation Systems Part II
Monday, 21 April 2008, 3:30 PM-5:30 PM, Standley I

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