Environment

Cloud study shows how planes can significantly boost rainfall in their path

Cloud study shows how planes c...
A new study shows how planes can affect clouds, boosting rainfall up to 10 times in narrow bands
A new study shows how planes can affect clouds, boosting rainfall up to 10 times in narrow bands
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In this radar image, a streamer of heavier rainfall can be seen as a yellow line on the left side, following the flight path of a plane as it approached Helsinki Airport
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In this radar image, a streamer of heavier rainfall can be seen as a yellow line on the left side, following the flight path of a plane as it approached Helsinki Airport
A new study shows how planes can affect clouds, boosting rainfall up to 10 times in narrow bands
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A new study shows how planes can affect clouds, boosting rainfall up to 10 times in narrow bands

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.

In this radar image, a streamer of heavier rainfall can be seen as a yellow line on the left side, following the flight path of a plane as it approached Helsinki Airport
In this radar image, a streamer of heavier rainfall can be seen as a yellow line on the left side, following the flight path of a plane as it approached Helsinki Airport

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.

Source: American Geophysical Union

6 comments
bql
actually planes are spraying coal fly ash in the skies as a way of getting rid of dangerous industrial waste. thats why theres so weird weather in last years. and thats why we dont have a normal winter now.
F. Tuijn
This story suggests that it is caused by the tip vortices.
Ralf Biernacki
"Between December 2008 and January 2018, the team found a further 17 days where the streamers appeared." 17 appearances, for all the air traffic in and out of Helsinki Airport over nine years? And these elusive trails are not much wider than an airplane. Hardly seems like a significant effect, more of an atmospheric curiosity.
Nobody
Not much data to prove a preconceived theory. Only 17 events over 10 years and claiming a boost of rain and snow by 10 times. What about other factors like the condensation of the vapor trails? A gallon of jet fuel can make several gallons of water as a product of combustion. Just the thermodynamics of combustion to the formation of ice crystals could have a significant effect. The reflectivity of the white vapor trails can also reflect sunlight and produce cooling. I have seen totally clear days with the sky full of vapor trails resulting in a high overcast and reduced sunlight. If this is Dimitri's research paper, it is definitely lacking credible data. When I was in college, the most important part of a scientific research paper was the section about possible errors. There are far too many assumptions and possible variables not even mentioned to reach any conclusions, only to suggest further studies.
paul314
Vortices would certainly seem to make sense. As pilots know who have experienced clear air turbulence, vortices can get really big and carry a lot of energy. The would also be mixing air from adjacent layers with different-enough conditions that a cloud could be pushed past its tipping point.
Vernon Reuben
I read with some interest your post on aircraft and how they can impact on our weather.

I do not support geoengineering but what happened over the past week (November 10 to 15) in the Kwazulu-Natal province of South Africa was nothing short of spectacular.

We have come off a week of torrential downpours, tornados, severe thunderstorms and powerful surface winds which caused destruction, flooding and several deaths.

While these events have brought with it tragedy and destruction in isolated parts of the province I also made some carefully observations together with some work colleagues.

We watched with interest as aircraft already flying at high altitude were emitting large plumes from their wingtips. These could not have been contrails but chemtrails and the aircraft where flying at about 50-60 thousand feet creating striated patterns in the atmosphere above.

This must have gone on for a few days prior to the heavy rains and storms that followed.

Interestingly it does appear that this form of cloud seeding does work exceptionally well producing average to above average rainfall over certain areas and with moisture laden clouds blowing onto the coast it only helps to enhance rainfall over the province.

We probably had some of our best rainfall for the month of November in years having just come out of a 4 year drought.

If this is what can increase rainfall and crop yields then it is certainly the way to go.

The only uncertainty is how damage from storms, flooding and casualties can be avoided.

At the moment many areas of South Africa are experiencing crippling drought and water shortages.

Hopefully cloud seeding could help us at this time.

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