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°C, while the wings of jets can lower the temperature by 20°C. If a cloud is already in the area of -10 to -20°C, 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. On the periphery of the updraft, a corresponding downdraft causes evaporation of the water in the cloud, which gradually expands the hole. This self-sustaining cycle should cause the hole to expand for up to an hour after the aircraft passes, and vastly expands the area affected by the aircraft's passage.

The researchers looked at satellite and radar imagery to see whether they could spot the equivalent of what their model was showing. Could they ever. Radar near a Denver airport showed lines of snow formation that followed curved paths typical of airliners. Clouds over the continental US showed extended trains of holes and channels that followed flight paths verified on air transportation records. "Some holes reached lengths of >100 km and were detected for four or more hours," the authors note. From the point where they were detected on radar, the holes would double their area within a half hour, and continued to expand out to a full hour.

The authors don't expect that the formation of snow is significant enough to do much more than cause small amounts of localized snow in the immediate vicinity of the airport. But the satellite imagery suggests that heavy air travel can cause a measurable drop in the total area covered by clouds under the right conditions. Along with the earlier results, the new work indicates that, although aircraft only disturb a small area of the atmosphere, the disturbances they cause can persist and propagate long enough to have an unexpectedly large impact on our planet.

Science, 2011. DOI: 10.1126/science.1202851 (About DOIs).

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