From the American Geophysical Union weekly highlights:
Estimating climate effects of contrails
In a new study, Voigt et al. directly measured ice particle sizes and numbers in 14 contrails from 9 different aircraft of the present-day commercial fleet, including the largest operating passenger aircraft. They obtained an extensive data set of contrails from which they determined the contrail optical depth, a measure of how much light is attenuated by these man-made clouds.
They use their measurements to estimate that the radiative forcing of line-shaped contrails is about 15.9 milliwatts per square meter, which represents a small positive contribution to the anthropogenic global warming. Yet an expected doubling of aircraft passenger transport within the coming two decades will enhance contrail effects on the atmosphere. The detailed contrail measurements will help modelers working to assess the actual and future impact of aviation on climate.
Source: Geophysical Research Letters, doi:10.1029/2011GL047189, 2011
http://dx.doi.org/10.1029/2011GL047189
Title: Extinction and optical depth of contrails
Authors: C. Voigt: Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, Germany; and Institut für Physik der Atmosphäre, Johannes-Gutenberg University, Mainz, Germany;
U. Schumann, P. Jessberger, T. Jurkat, and A. Petzold: Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, Germany;
J.-F. Gayet: LaMP, University Blaise Pascal, Clermont-Ferrand, France;
M. Krämer: IEK-7, Institute for Energy and Climate Research, Forschungszentrum Jülich, Jülich, Germany; T. Thornberry and D. W. Fahey; Chemical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA.
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Some basic science behind contrails from http://cimss.ssec.wisc.edu/wxwise/class/contrail.html:
The condensation trail left behind jet aircrafts are called contrails. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust. Cloud formation by a mixing process is similar to the cloud you see when you exhale and “see your breath”. The figure below represents how saturation vapor pressure varies as a function of temperature. The blue line is the saturation vapor pressure for ice as a function of temperature (in degrees Kelvin). Air parcels in the region labeled saturated will form a cloud. Imagine two parcels of air, A and B as located on the diagram. Both parcels are unsaturated. If B represents the engine exhaust, then as it mixes with the environment (parcel A) its temperature and corresponding vapor pressure will follow the dotted line. Where this dotted line intersects the blue line is were the parcel becomes saturated.
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NOTE: Any commenters that discuss or link to “chemtrail” discussions will have the comment automatically deleted. No exceptions, and no, I don’t care if it upsets you – Anthony
Haydenlee says:
June 23, 2011 at 11:20 pm
This from a layman which probably is now evident – it’s been my understanding that vapor trails are created not by jet engine exhausts (hence contrails from piston engined planes) but by pressure difference between the top and bottom of a wing created by slip stream spilling (spiraling) around the wing tip. When the atmosphere has the right combination of moisture and temperature, the contrail is formed by the sudden drop in air pressure. Can anyone put me right?
Vapor trails are caused by the combustion exhaust gases which can contain fairly high water vapor pressures, these condense under the appropriate conditions as shown above. If a plane flies through just subsaturated air then wing-tip vortices can also cause a vapor trail. In this photo you can also see condensation in the low-pressure region above the wing.
>> Phil. says:
June 24, 2011 at 6:45 pm
But all things are not equal, solar irradiation will typically have wavelengths less than the particle size and give Mie scattering which will scatter in all directions whereas the IR has wavelengths significantly larger than the particle size which will give Fraunhofer scattering which will be predominantly in the forward lobe. <<
My point was that any scattering will give the same end result if the layer is thick enough.
As far as size is concerned, are the ice crystals (or water droplets in the case of clouds) really significantly smaller than 10 microns?
I guess I should Google it myself. Answer, water droplets are 6-14 microns, cirrus ice crystals 5-40 microns.
http://www.sciencemag.org/content/295/5556/834.abstract
http://www.inscc.utah.edu/~tgarrett/Publications/Icescattering/GarrettGRL2003.pdf
How would the presence of barium and aluminium affect the radiative forcing of a contrail, perhaps as part of a weather modification programme?
Tom_R says:
June 24, 2011 at 9:13 pm
>> Phil. says:
June 24, 2011 at 6:45 pm
My point was that any scattering will give the same end result if the layer is thick enough.
As far as size is concerned, are the ice crystals (or water droplets in the case of clouds) really significantly smaller than 10 microns?
I guess I should Google it myself. Answer, water droplets are 6-14 microns, cirrus ice crystals 5-40 microns.
Or read the paper referred to in the OP which gives 2.9 microns.
>> Phil. says:
June 25, 2011 at 5:13 am
Or read the paper referred to in the OP which gives 2.9 microns. <<
That's the effective size the authors measured for contrails. But my comments in the post you originally quoted were about clouds.
As an old fighter pilot, we used to brief the contrail levels (nominally 30-35000 ft over the UK) and avoid flying in them to negate an adversary’s visual pickup. Now, as a B747 Captain, I sit happily in the trail levels as these are the fuel-based optimum levels, for up to 12 hours. If (?) things are as bad as our great thinkers (!!!!!) propose then how about going back to the fighter pilot mentality and avoid the (bad) levels – and burn more nasty jet fuel?
@ur momisugly al gallacher
Now that is interesting. Nice to have a bit of practical knowledge thrown in.
Since my post from yesterday apparently didn’t make it I’ll repeat it:
Tom_R says:
June 25, 2011 at 6:30 am
>> Phil. says:
June 25, 2011 at 5:13 am
“Or read the paper referred to in the OP which gives 2.9 microns.”
That’s the effective size the authors measured for contrails. But my comments in the post you originally quoted were about clouds.
Perhaps you should have stuck to the subject.
Thanks for the video Gee Willikers. I find myself almost unable to watch BBC documentaries these days. They are so overblown and ridiculous they almost parody themselves. I find their excessively emotive propagandizing very annoying. That is a shame because there is often an interesting fact or two in them albeit buried deeply in the rest of the garbage. Indeed watching this prompted a number of interesting thoughts.
1. Remember that fleet of wind powered robot ships that were supposed to spray salt water into the air to make clouds whiter and protect us from global warming? Looks like the idea might work, but why bother with the robot ships – just catch a plane.
2. CO_2 greenhouse physics by itself absolutely cannot account for more than 1.2 degrees per doubling of CO_2 without some kind of positive feedback mechanism operating. That mechanism was supposed to be additional greenhouse warming from higher levels of water vapor resulting from increased evaporation in a warmer world. But this whole global dimming scenario seems to completely destroy that as a working hypothesis. If pan evaporation rates have declined significantly in the 20th century then there cannot have been any positive feedback from water vapor operating over this time period. Unless an alternative positive feedback mechanism can be cooked up this leaves the CO_2 greenhouse effect acting by itself, and that effect is therefore far too small to explain the observed warming in the second half of the 20th century. That warming must have been caused by something else. The dramatic doomsday predictions at the end of the documentary nicely ignore the fact that global dimming has just destroyed the only proposed physical mechanism by which it is envisaged that CO_2 could cause this level of warming.
3. This will also cause problems for the solar warming people. The global dimming caused by contrails involves and interferes with the exact same mechanism by which changes in solar activity are supposed to effect the climate. The idea is that solar activity changes the cosmic radiation flux effecting rates of cloud seeding. The good news for this theory is that rates of cloud seeding do indeed seem to have a significant impact on climate making the mechanism more plausible. The bad news is that temperature increased anyway during a period where cloud seeding rates had apparently been significantly increased due to contrails. Why? Something else must have done it. Furthermore if we are directly changing cloud seeding rates ourselves then we cannot expect this mechanism to work the same way in future as it did in the past. So the solar Maunder minimum type period we may be looking at now might not have quite the impact some people expect it to have.
So where does this leave us? Perhaps merely with an increased appreciation of how much there is about climate we don’t understand.
Cynical S;
AFAIK, the Svensmark GCR hypothesis speaks of low-level clouds, not high cirrus type. Contrails are way too high to be low-level cloud seeding, and are far less effective reflectors.