By John Timmer, Ars Technica
We’ve seen recently that air travel can have an oversized impact on the atmosphere, at least relative to emission of things like greenhouse gasses, because they seed clouds that can persist for hours. Now, researchers have taken a detailed look at what happens when aircraft fly through clouds that already exist. Under many circumstances, it turns out that the aircraft have the opposite effect, causing pressure changes that trigger the formation of large holes in the cloud, with the missing water falling out as snow.
Holes in clouds, like the one shown above, have been associated with the passage of aircraft since the 1940s, but it hasn’t been clear how significant this process is. By the 1980s, researchers had found that propeller-driven aircraft produce large pressure changes in their wake, which produce a lot of ice crystals if they pass through supercooled clouds.
More recent work showed just how large this effect can be: Propellers can induce temperature drops of up to 30 degrees Celsius, while the wings of jets can lower the temperature by 20 degrees Celsius. If a cloud is already in the area of -10 to -20 degrees Celsius, that means the airplane should cause ice to crystalize out of clouds.
The new paper shows what happens as a result of these crystals. Modeling indicates that the heat released by ice formation causes an updraft in the immediate vicinity of where an aircraft passes. This carries most of the ice crystals back upward, with the exception of the heaviest, which fall through the updraft as snow.