Aircraft are a major contributor to CO2 emissions, but it's not just their pollution that can affect the weather and climate – they've been known to mess with the clouds as they pass through or over them. A new study has found that planes could be boosting rainfall and snowfall by up to 10 times, and examined the microphysics behind it.

The study began when University of Helsinki researcher Dimitri Moisseev noticed something odd in radar data: Very narrow, straight streamers of clouds seemed to be producing heavier rain or snow than larger surrounding areas. Their location gave a clue to what was causing them – they seemed to mostly point towards the nearby Helsinki-Vantaa airport.

To examine whether planes could be creating the phenomenon, the researchers investigated 11 years' worth of weather radar data gathered at the university. Between December 2008 and January 2018, the team found a further 17 days where the streamers appeared.

Next, they cross-checked the data against archived flightpath records going back to 2011, and found that in most cases, aircraft had passed within 2 to 10 km (1.2 to 6.2 mi) of the streamers.

"The intensified precipitation basically follows the track of an airplane above the cloud," says Moisseev. "It could extend over hundreds of kilometers, but the cross-section would be maybe 100 m (330 ft). So it's a very narrow, long feature."

So how could planes cause this kind of weird weather pattern? The team found that they're probably related to similar events like "hole punch clouds," where planes taking off or landing can create surprisingly circular holes as they pass through the cloud layers. Even though the holes cause gaps in rain and the streamers cause increases in it, the two phenomena may be the result of the same physics.

Supercool clouds

Contrary to popular belief, water can exist as a liquid well below the usual freezing point – at least, under specific circumstances. Water droplets can crystallize into ice more easily if there's a surface for it to latch onto, such as dust particles. In clouds with few impurities, water droplets can stay liquid right down to -40° C (-40° F).

That is, until a plane comes along and stirs things up. As the aircraft passes through a cloud of these supercooled water droplets, changes in the air pressure can freeze the droplets, which in turn freeze others around them. As this chain reaction spreads out and the ice falls away, it can create the hole punch cloud formation.

But that should only affect the clouds when the plane first flies through them, right? How would planes cause the streamers, hundreds of kilometers long, when they're flying well above the cloud line?

The data in the new study suggests that the effect starts much higher up. Layers of supercooled water vapor float around the same altitude that planes approach the airport from, so when a plane passes through, ice crystals can fall from this layer into a lower cloud layer that is already raining or snowing. That feeds more ice into the cloud and boosts the precipitation along a narrow band behind the aircraft.

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

The team says that understanding the phenomenon better could help improve the weather forecasts around airports, where that's both incredibly important and most susceptible to swings caused by air traffic.

The research was published in the Journal of Geophysical Research: Atmospheres.

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