Saturday, July 2, 2011

Airplanes "can make it snow"

Airplanes flying through super-cooled clouds around airports can cause condensation that results in more snow and rain nearby, according to a new study.

The correct conditions for this inadvertent weather modification occur about five per cent of the time — but 10-to-15 per cent in winter — according to Andrew J. Heymsfield of the National Center for Atmospheric Research in Boulder, Colo., lead author of the study appearing in Friday's edition of the journal Science.

Aircraft take off into the wind, he noted, so if they are generating extra ice particles upwind of an airport, the result can be snow right on the airport. That might mean planes will require more de-icing, he said, though other researchers weren't so sure.

The team was investigating holes or canals that are sometimes seen drilled in clouds after an airplane has passed through.

Studying six commercial airports, including the one in Yellowknife, they found that increased snow and rainfall occurs in areas where the unusual cloud holes appear, usually within 100 kilometres of the airport. Places farther away from an airport are more likely to be at higher altitudes, above the clouds.

The added rain or snowfall occurred when conditions in the clouds were super-cooled. That means the clouds were made up of water droplets that were colder than freezing, but which had not yet frozen.

Water in the atmosphere can remain liquid at temperatures below freezing if it doesn't have any type of nucleus to freeze onto, such as bits of dust or salt. It will freeze without a nucleus when it gets very cold, however — about -15 C. (read more)