Dance for Rain, Don’t Seed Clouds Finds New Research

israel rain danceCloud seeding not more effective than chance (or prayers?) finds new research.

Prayers are answered as rain starts falling in the Middle East –– but not before hot, possibly climate change related dry spells have helped spread wildfires in Lebanon and Israel. But those looking to cloud seeding to ignite a rain: better do a rain dance. Cloud seeding, a weather modification process designed to increase precipitation amounts by dispersing chemicals into the clouds, does not work, has found Tel Aviv University researchers.

In the most comprehensive reassessment of the effects of cloud seeding over the past fifty years, new findings from Prof. Zev Levin, Dr. Noam Halfon and Prof. Pinhas Alpert have dispelled the myth that, at least in Israel, cloud seeding is an effective mechanism for precipitation enhancement. The findings were recently reported in Atmospheric Research.

During the course of their study, the researchers looked over fifty years’ worth of data on cloud seeding, with an emphasis on the effects of seeding on rainfall amounts in a target area over the Sea of Galilee in the north of Israel. Using a comprehensive rainfall database they compared statistics from periods of seeding and non-seeding, as well as the amounts of precipitation in adjacent non-seeded areas.

They were able to show that increments of rainfall happened by chance. For the first time, they were able to explain the increases in rainfall through changing weather patterns instead of the use of cloud seeding.

Most notable was a six-year period of increased rainfall, originally thought to be a product of successful cloud seeding. Prof. Levin and his fellow researchers showed that this increase corresponded with a specific type of cyclone which is 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.

Despite being relatively expensive, there are more than 80 cloud seeding projects around the world, according to a recent World Meteorological Organization report. Unfortunately, most are operational projects with no scientific evaluation of success or failure.

In Beijing, China, for example, 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, thus increasing water levels in reservoirs.

Like clinical studies

However, Prof. Levin notes thus far there is no proof that this method is successful. It is important to understand that in order to determine that cloud seeding enhances rainfall, a complicated statistical evaluation needs to be conducted and comparison needs to be made with similar areas that are not seeded.

One could take an analogy from the testing of new drugs. For a drug to be approved a comprehensive test needs to be conducted in which part of the sampled population is given a placebo. The same is with cloud seeding, for comparison; some of the sampled clouds need to be unseeded.

The only probable place where cloud seeding could be successful, Levin says, is when seeding is performed on orographic clouds, which develop over mountains and have a short lifespan. In this type of cloud, seeding could serve to accelerate the formation of precipitation.

Image via rogerblackwell

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