Given that it’s a practice invented in the 1940s, it may seem surprising that to this day, no one really knows if cloud seeding is worth the hassle. The controversial practice involves forcing clouds to expel rain by ‘seeding’ the atmosphere with chemicals such as silver iodide or dry ice. This process can force lightweight water vapour to form ice crystals which are then heavy enough to fall as rain or snow. The idea is that more rain can be encouraged to fall than might do otherwise.
Yet, despite the fact that 56 countries had cloud seeding operations in 2016, according to the World Meteorological Association, and the fact that the United Arab Emirates recently launched a £3.6m programme to research rain enhancement, the evidence for its efficacy is limited. Previous attempts have estimated that the process leads to anything between 0 and 50 per cent more snowfall.
A team of researchers from the University of Colorado Boulder are trying to be more precise. Earlier this year they carried out three injections of silver iodide into a natural cloud formation over western Idaho. They then used a new technique, which involved peering into the clouds using radar, to measure how much additional snow fell as a result. Based on the team’s calculations, snow fell for about 67 minutes, dusting roughly 900 square miles of land in about a tenth of a millimetre of snow. Though it doesn’t sound like much, in total, the three events produced about 282 Olympic-sized swimming pools worth of water.
‘We tracked the seeding plume from the time we put it into the cloud until it generated snow that actually fell onto the ground,’ said Katja Friedrich, an associate professor in the Department of Atmospheric and Oceanic Sciences. ‘If we hadn’t seeded these clouds, they would not have produced any precipitation.’
This doesn’t mean that all seeding events will have the same results, or be cost effective. Unfortunately, every cloud is different and interacts with aerosols in different ways. Some clouds may have areas that aren’t seed-able at all. Nevertheless, the team hope that their research will make it easier to perform cost benefit analyses in the future. ‘Everyone you talk to will say even if you can generate a little bit more snow, that helps us in the long run,’ said Friedrich.