Dust anybody?: Hurricane Katrina enters the Gulf of Mexico on 27 August 2005. A hurricane builds up strength as it passes over warm water. In this image, which is based on data from NASA's Aqua satellite, the ocean is colored according to its 3-day average surface temperature, with the warmest water being red.

Credit: NASA/SVS

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TRIESTE, Italy: Seeding hurricanes with microscopic dust can sharply reduce their force, says a study which calculated that the technique might have spared New Orleans from disaster in 2005.

The findings were presented this week at the European Conference on Severe Storms held in Trieste, Italy.

In computer simulations, scientists from the Hebrew University of Jerusalem, Israel, showed that sowing tiny moisture-seeking particles into the lower reaches of a hurricane could prevent the formation of rain and reduce temperatures, starving the storm of its energy source.

Starving a storm

The process "creates clouds with a large number of small drops that fall very slowly, floating with air molecules, and are less likely to collide with each other and coalesce into rain drops," said Daniel Rosenfeld, part of the team who developed the concept.

Hurricanes – or cyclones – are powerful, swirling storms up to 800 km wide that begin over tropical seas with a surface temperature greater than 26.5 °C. The warm seawater evaporates and is absorbed by the surrounding air.

Rosenfeld first tested his computer model in a control run to see whether the simulation would match Katrina as it really happened, which it did.

When he factored in the effect of cloud seeding – taking into account the impact of sea spray, which would reduce the desired effect – the radius of hurricane-force winds shrunk by at least 25 per cent, with wind speeds reduced throughout the hurricane (hurricane-force winds begin at 119 km/h and can reach speeds of more than 300 km/h).

"That [reduction] would affect mainly the sea surge, which means less rising of the water, which might have made the difference in New Orleans," Rosenfeld said. The simulated path of the weakened storm curved north as compared to Katrina, and would have made landfall about 200 km east of the city, according to the simulation.

To put this seeding technique into practice it would take five to 10 Lockheed C-130s cargo planes to disperse 200 tonnes per hour of particles so small (less than one millionth of a metre across) that they would be emitted in the form of smoke.

The planes would be hundreds of kilometres from the eye of the hurricane, and thus out of harm's way, said Rosenfeld.

Stormfury revisited

Cloud seeding has been more typically used to attempt to make or impede precipitation, but extending the practice to hurricane control is not new.

In a project called Stormfury, the U.S. government ran a series of experiments from 1962 to 1983 that attempted to decrease hurricane force by artificially stimulating convection currents outside the wall, which encases the eye of the storm. The idea was to expand the size of the eye – typically 15 to 65 km in diameter – and thus slow the destructive winds that swirl around the eye wall.

Tests were done on four hurricanes before the technique was abandoned. Rosenfeld was inspired to try again after observing that a "heavy load of small aerosols" – in other words, smoke from forest fires – can prevent warm rain from tropical clouds.

"I tried to fix some of the problems that prevented Stormfury from working," he said. One of the co-authors of the new study, William Woodley, flew into hurricanes during the 1970s as part of the earlier effort.

Putting this hurricane-taming concept into practice, however, will take years of additional research and experimentation, said Rosenfeld. But the fact that another research team in the United States, working independently, came up with the same idea at about the same time suggests that it may just hold water.