Geoengineering: Clouds over troubled waters WHEN drought threatened them, some groups of Native Americans used to hold rain dances. These colourful rituals, involving turquoise ornaments to represent water and feathers to stand in for the wind, were to urge the gods to send a storm. Now such dances are reserved for tourists - and rain-making has become a science, involving seeding clouds by dropping frozen carbon dioxide (dry ice) or silver iodide into them from aircraft. This works, though it is not widely practised outside China because the benefit is rarely reckoned to exceed the cost. A new study, however, suggests a similar technique might have great benefits indeed, by reducing the devastation caused by a type of weather no one welcomes: hurricanes. Cloud-seeding works because the substances employed gather molecules of water vapour around themselves to form droplets. With enough such condensation, a cloud can be encouraged to produce more rain than it otherwise would have done. Rather than rain, though, it is the idea of fiddling with clouds" brightness that interests Alan Gadian of the University of Leeds, in Britain, and his team. A hurricane forms when intense sunlight heats the surface of the sea, driving both evaporation and convection. As the warm, moist air thus produced rises, it cools. The consequent condensation of the water vapour it is carrying heats the air further (the change of state from gas to liquid sheds latent heat), causing it to continue rising. So the process feeds on itself to generate powerful winds and heavy rainfall. In most parts of the world the surface of the sea is not warm enough for hurricanes to form. Sometimes that is because of its latitude, but sometimes it is because it is covered by low cloud layers, which reflect sunlight. Roughly 30% of the ocean is so covered. And it was this fact that made Dr Gadian and his colleagues wonder what would happen to hurricanes if that effect were amplified by making the clouds brighter. This should be possible by making seeds that increase the number of water droplets in the clouds. Instead of using aircraft-borne chemicals as seeds, the researchers propose spraying seawater from unmanned, wind-powered, satellite-controlled vessels that would cruise the latitudes where hurricanes form - since droplets of brine have a similar effect to dry ice and silver iodide. In the absence of a multi-billion-dollar research budget and the necessary permissions, however, their ships have sailed only through the processor of a computer. But the results are still illuminating. The team report in Atmospheric Science Letters that brightening virtual clouds around the world in this way caused ocean temperature in regions where hurricanes form to drop by as much as 4°C. If that happened in the real world, it would heavily suppress the intensity of these storms. Pulling this trick off, however, required the deployment of 2,000 virtual spray ships, and also interfered with rainfall patterns in a way that would not be appreciated by the world's farmers. Such an armada might not, however, be necessary to produce a more modest but still useful effect. Simulating the activities of spray ships in just the three areas of the world with plenty of low-level marine clouds resulted in a temperature decrease of only 0.1°C. Though that did not make the situation any better than it is now, it was enough to stop storms becoming even more powerful as global carbon-dioxide levels rose, and ocean temperatures rose with them. Such grand-scale geoengineering, as it is known, would not merely be a technical challenge to implement in reality. It would be a political and legal nightmare, too. But even if it remains safely locked away in a computer, it gives pause for thought about the extent to which people might influence the climate with a specific end in view. It would make a change.