Recently both Russia and China have claimed to be able to use cloud seeding to increase rainfall and snowfall, or change the location of where it falls. In the past, snow-making experiments have been carried out in North American ski resorts in the past with little evidence of success. So how have the Russian and Chinese scientists achieved this feat and what evidence is there that it is in fact due to cloud seeding?
The seeding method used is to add tiny particles of silver iodide to the clouds and there is solid science behind this method. At temperatures a few degrees below zero degrees centigrade, clouds consist mainly of supercooled water droplets. These clouds can be quite stable, but silver iodide has an ice-like structure and it will cause a few of these water droplets to freeze. Once you have ice particles mixed in with the supercooled droplets, these crystals grow rapidly to form snowflakes, causing the cloud to precipitate. The effect is that the water is released from the cloud.
This form of cloud seeding is not new - It has been used since the 1960s in the western USA to try to make rain, as well as being used in Israel in the past. The difficulty is it has always been hard to prove whether the cloud would have rained naturally if it hadn’t been seeded. Even tests where seeding lots of clouds takes place and un-seeded clouds are used as controls haven’t produce statistically significant results. Therefore, if it works at all it can’t be hugely effective.
So although the underlying science behind the technique seems to be sound, what is presently wrong with the technique?
We at Manchester University have been flying the UK atmospheric research aircraft in clouds that may or may not produce rain or snow, to investigate effective conditions for cloud seeding in different environments. Last winter, we were flying in low clouds with temperatures just below freezing and we did not manage to seed any of the clouds and they were left entirely natural. We found that on many occasions these clouds already contained a mixture of supercooled water droplets and ice crystals, and the ice crystals were growing and falling out as snow. However this wasn’t on account of our seeding efforts.
Interestingly, the origin of these ice particles seemed to be due to freezing caused by particles of dust, ash and organic material carried up into the cloud (some of which can be attributed to pollution). We found that once some ice is formed there is a powerful secondary ice particle production process which occurs at about minus six degrees Celsius. This produces lots of ice crystals, meaning that even if the clouds had been seeded with silver iodide, it would have had little effect. Snow was being produced very effectively and quite naturally anyway.
Interestingly, when we flew higher into clouds, well above the ground and away from a lot of the particulate material, we did find clouds at colder temperatures, as cold as -30C that were made up entirely of supercooled water droplets so maybe seeding these would have had an impact.
So does cloud seeding work? Well our studies indicate that in many clouds that produce lots of snow it does not seem to, because there is plently of natural ice already. However, I don’t completely dismiss it as a method – I do believe it’s possible it can be effective in some clouds in the right conditions and at the right temperature. Nevertheless, I feel some of the stronger claims made recently need further verification, before we herald this as a breakthrough in scientists’ ability to manipulate natural weather cycles.
Professor Tom Choularton is Head of the School of Earth, Atmospheric & Environmental Sciences is supporting the Science: [So what? So everything] campaign, which aims to highlight the leading UK science research that will shape the future of Britain.
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