To encourage rainfall, scientists often use "cloud seeding," a weather modification process designed to increase precipitation amounts by dispersing chemicals into the clouds, however, a group of researchers in Israel proved of late that it may not be as effective as it had been thought.

A study conducted at Tel Aviv University's (TAU) Department of Geophysics and Planetary Sciences now reveals that cloud seeding with materials such as silver iodide and frozen carbon dioxide may not be as effective as it had been thought and hoped, according to a press release sent to Xinhua on Tuesday.

In a comprehensive reassessment of the effects of cloud seeding over the past 50 years, a research team headed by Pinhas Alpert dispelled the myth that seeding is an effective mechanism, the press release said.

Israel is a classic example of a country struck by constant droughts. That is one reason why TAU researchers placed an emphasis on the effects of seeding on rainfall amounts in a target area over the Sea of Galilee in northern Israel.

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

In attempting to find the reason behind a six-year period of particularly generous rainfall, originally thought to be the product of successful cloud seeding, Alpert and his team showed that it corresponded with a specific type of cyclones which are consistent with increased rainfall over mountainous regions.

Their findings, recently reported in Atmospheric Research, concluded that changing weather patterns were responsible for the higher precipitation levels during that time.

The study, said Alpert, may be useful in the investigation of cloud seeding in the United States and other regions. According to a recently-published World Meteorological Organization report, there are more than 80 cloud seeding projects held around the world, including California's Sierra Mountains and in Wyoming.

Until a more effective technique of forcing rain showers is found, the study does give little hope for what is available. The only probable place where cloud seeding could be successful, researchers say, is when performed on orographic clouds, which develop over mountains and have a short lifespan.