December 9, 2010, Updated September 13, 2012

New research from Israel shows that changing weather patterns – not the weather-modification process of cloud seeding – is what makes the rainfall.


Photo by Kobi Gideon/Flash90.
Scientists have discovered that cloud seeding doesn’t increase rainfall.

Rain is such a rare and precious resource in many parts of the world that scientists developed the weather-modification process called cloud seeding, which disperses chemicals into the clouds to increase precipitation. New research from Israel is now calling that practice into question.

In the most comprehensive reassessment of the effects of cloud seeding over the past 50 years, new findings from Prof. Pinhas Alpert, Prof. Zev Levin and Dr. Noam Halfon of Tel Aviv University’s Department of Geophysics and Planetary Sciences have dispelled the myth that seeding is an effective mechanism for precipitation enhancement.

The research findings, recently reported in the journal Atmospheric Research reveal that the common practice of cloud seeding with materials such as silver iodide and frozen carbon dioxide may not be as effective as had been hoped.

It all depends on the weather

During the course of his study, Alpert and his colleagues examined more than 50 years’ worth of data on cloud seeding, focusing on the effects of seeding on rainfall amounts in a target area over the Sea of Galilee in northern Israel.

The research team used a comprehensive rainfall database and compared statistics from periods of seeding and non-seeding, as well as the amounts of precipitation in adjacent non-seeded areas.

“By comparing rainfall statistics with periods of seeding, we were able to show that increments of rainfall happened by chance. For the first time, we were able to explain the increases in rainfall through changing weather patterns, instead of the use of cloud seeding,” reports Alpert.

Most notable was a six-year period of increased rainfall, originally thought to be a product of successful cloud seeding. Alpert and his fellow researchers show that this increase corresponded with a specific type of cyclones which are consistent with increased rainfall over the mountainous regions. They observed a similarly significant rain enhancement over the Judean Mountains, an area which was not the subject of seeding.

Research should be useful in the US

The researchers conclude that changing weather patterns were responsible for the higher amount of precipitation during these years. Their research method may be useful in the investigation of cloud seeding in the US and other regions.

Despite being relatively expensive, there are more than 80 cloud seeding projects around the world, according to a recent World Meteorological Organization report. In Beijing, China, for example, Alpert notes, a large amount of chemical particles were introduced to the clouds to inhibit precipitation – a process called ‘overseeding’ – to limit rainfall during the 2008 Olympics.

Seeding is also used in the Sierra Mountains of California and in Wyoming to try to increase precipitation in the mountains, in order to increase water levels in reservoirs. However, he says, there is no proof that this method is successful.

The only probable place where cloud seeding could be successful is in orographic clouds, which develop over mountains and have a short lifespan. According to Alpert, in this type of cloud, seeding could serve to accelerate the formation of precipitation.

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