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.
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