Science Questions

Do contrails cool the atmosphere?

Sat, 20th Aug 2011

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Jonathan Urbach, via Facebook asked:

I read once that airplane contrails may cool the climate and that this effect was observed following the grounding of planes after the September 11 attacks. Could this effect be the basis for climate change mitigation?


Dave -  I think there have been some studies done on this - there's one 3 or 4 years back - and I think they can either increase the temperature or decrease it, depending on where they are. If you're somewhere very, very sunny then they can actually reflect more heat out, than they reflect back in.  I think on average, they reckon, especially in places like the UK, they tend to actually heat up the Earth more because they are acting as an insulating blanket, as cloud does, and so they actually, if anything, increase the temperature.


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The actual Nature article

So, very basically, it's colder at night and hotter during the day. imatfaal, Wed, 24th Aug 2011

I think that would be the case in the absence of contrails, so contrails have the opposite effect. Geezer, Wed, 24th Aug 2011

I think that would be the case in the absence of contrails, so contrails have the opposite effect.

details details.... imatfaal, Thu, 25th Aug 2011

I think the idea is this.

During the day, the contrails block the sunlight coming in (reflect it back out to space).  Somewhat like you would experience on a cloudy day where it is much cooler at the surface.  It may, however, become slightly warmer at the altitude where the jets are flying.

During the night, the contrails block the IR from leaving, and dissipate and reflect it back down, like a blanket. Thus, causing a warming effect.  This is similar to what one experiences on cloudy nights/mornings. 

On average, half the day is night-time.  Half the day is day-time, so it probably averages out to little overall effect.

However, away from the equator, the day/night cycles are not evenly distributed.  So, for example, over the poles one might experience significant warming over the winter, and cooling over the summer.  But, increasing moisture in the atmosphere over the poles may also give a slight positive increase in snow when the cold air has low moisture carrying content. CliffordK, Thu, 25th Aug 2011

Perhaps there is scope to modify the path of a landbound hurricane. Environmental conditions permitting multiple contrails could be arranged to divert an oncoming hurricane a little like the opposite of the role played by a sweeper in curling. The idea is that all things being equal (such as prevailing wind conditions,) a hurricane may tend to move toward warmer water (or away from cooler water). Contrails can cast enormous shadows which is just as well given the size of weather systems such as hurricanes. I'm aware of several hurricane modification programmes but have never seen anything like this tried. When hurricanes do land fall the insurance bill can be enormous so as wacky as it sounds it might be worth a try?  Mootle, Sun, 16th Oct 2011

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