Powerful, low-frequency sound waves could be used to trigger rainfall in areas that suffer from drought, according to a study by researchers at Tsinghua University in Beijing.
In a weather manipulation experiment conducted on the Tibetan Plateau last year, the researchers said they recorded increases in rainfall of up to 17 per cent by pointing a giant loudspeaker at the sky.
“The total annual atmospheric water vapour resource in China is about 20 trillion tonnes. [But] only 20 per cent forms natural precipitation that reaches the ground, and the precipitation conversion rate in western regions is even smaller,” said the team led by Professor Wang Guangqian from the university’s State Key Laboratory of Hydro-science and Engineering.
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The sound energy might have changed cloud physics, but the cause of the phenomenon would require further investigation, the researchers said in a peer-reviewed paper published in Scientia Sinica Technologica last week.
Unlike other rainmaking technologies, sound generation produced no chemical pollution and required no “airborne vehicles such as aircraft or rockets”, Wang said. “And there is the possibility of remote control with low cost.”
The experiment is likely to add fuel to the long-running debate in China on the feasibility and environmental impact of large-scale weather modification programmes.
Critics have accused Wang, who proposed the controversial Sky River project to increase rainfall across Tibet by intercepting wet air circulating over the plateau, of wasting taxpayers’ money. Others say that even if the sound stimulation method works, it would create noise pollution for the people and animals that live in the area.
Wang’s loudspeaker was powered by a diesel engine capable of compressing more than 30 cubic metres of air to about 10 times the atmospheric pressure at sea level. That was then used to fire the sound at clouds at a frequency of 50 hertz, which is barely perceptible to most human ears but at a volume of up to 160 decibels, or about the same noise level as a jet engine running at full speed.
When the sound waves reached the cloud – about 1,000 metres (3,300 feet) above ground – their strength would drop by 30 decibels. Radar signals revealed significantly more water droplets were formed under the sound blast.
Wang and his team believe the increase is caused by the oscillation and merging of smaller particles into bigger ones.
In the study, the rainfall was 11 to 17 per cent higher in areas within the device’s effective range – a radius of about 500 metres from the sound generator – than outside it.
Despite the findings, a researcher from the Institute of Atmospheric Physics under the Chinese Academy of Sciences in Beijing who asked not to be named said Wang’s two-hour experiment would have to be replicated many times to gather more data.
While there has long been speculation that rainfall might be linked to sound – many civilisations perform rain dances in times of drought – the person said there were no physical theories to support the idea.
“The subject remains more of myth than science,” he said.
Wang and his team could not be reached for comment.
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