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Aeroplane flying over snowy city

Aircraft ‘wring’ intense snow from frigid clouds, trigger heavy weather

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Aeroplanes passing over rain or snow and causing local changes in air pressure can intensify raining or snowing as much as tenfold, a Finnish study has found.

According to the study, these bursts of precipitation are a consequence of the aeroplane’s wings passing through supercooled droplets of water in clouds, above a layer of rain or snow. This can boost rain and snowstorms over airports as planes pass through layers of cloud upon ascent and descent.

The idea behind the study arrived when Dimitri Moisseev, a researcher at the University of Helsinki and the Finnish Meteorological Institute, discovered unusual “streamers” of intense rain and snow appearing against a background of lighter rain or snow near Helsinki-Vantaa airport. Moisseev and his colleagues, suspecting that this could be caused in some way by passing aircraft, reviewed 11 years of Helsinki weather data and identified 17 days with repeat cases of these streamers.

The researchers studied flightpaths near the airport and found that aircraft passed near the intense streamers in most of the cases.

“The intensified precipitation basically follows the track of an airplane above the cloud. It could extend over hundreds of kilometres, but the cross-section would be maybe 100 metres, so it’s a very narrow, long feature,” said Moisseev.

The phenomenon occurs in supercooled liquid clouds, which form in low and mid-level cloud layers: very pure water can remain liquid and condense into clouds under sub-zero conditions (as low as -40°C). As aircraft pass by, the local drop in air pressure and temperature (falling to below -40°C) can trigger the supercooled droplets of water to freeze into ice crystals. This causes more water droplets to freeze into larger ice crystals in a widening area around the aircraft. As the crystals begin to fall, they create an opening in the cloud.

The researchers suspect that the formation of these streamers may not be straightforward due to the path of an aircraft through supercooled clouds, as the data suggests that the starting height of intensified rain and snow is above the layer already precipitating. Their analysis suggests that aircraft-generated ice crystals probably fall from supercooled upper cloud layers into a lower layer, causing up to ten times more precipitation in turn. This model is supported by satellite data.

“The interesting thing about this feature is that it is caused by aircraft, but it is not caused by pollution,” said Moisseev “Even if there would be absolutely ecological airplanes, which don’t have any combustion, no fuel or anything, it would still happen.”

Observing these bands of extreme rain and snow – which can also occur naturally – could be useful for “nowcasting” rain and snow just hours into the future, helping airports prepare their take-offs and landings.

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