I live under the flight path which airplanes take after leaving Logan (Boston) airport. I frequently notice the vapor trails left by the planes. My question: how much water vaport do airplanes add to the upper atmosphere, and is this enough to affect the amount of sunlight reflected back into space?
The short answer is yes
Actually, they don’t ADD much water vapor. After all, they’re not carrying water up there to spray. What they DO add is condensed water, ie they make visible cloud trails from teh water vapor already present in the air. And they DO add the chemicals & particulates left over in the exhaust from combustion of their fuel.
Sure they do. Jet exhaust is part water. It is formed when jet fuel is combusted, so the hydrogen molecules were indeed carried aloft.
As you probably expect, it’s a lot more complicated than that. Later research shows that high-altitude exhaust may actually produce ozone (net), but other pollutants are problems not yet fully understood. The main issue isn’t volume of pollutants, anyway, so much as that they’re placed directly in the stratosphere, without having to diffuse their way up from the surface.
But yes, there’s effects, as in the combustion of any hydrocarbon.
Perhaps even more complicated than that! 
“What is the main purpose of the extensive chemtrail jet aerosol program and is this operation a permanent necessity? I ask myself these puzzling and frustrating questions over and over again but there seems to be no certain answer. …Whatever the purpose is of this covert operation, it seems as if it is intended to last for quite a while, if not indefinitely. The reason I am currently coming to this conclusion has to do with the intense subliminal messages being put forth by the media. If …”
http://www.rense.com/politics6/chemdatapage.html , e.g.
(Sorry; long day)
Even more complicated…
Read this little piece from a past Nova episode.
It’s not long, has detailed descriptions of what causes contrails to form and a small bit of data from when there were no contrails immediately following the grounding of nearly all jet aircraft after 9-11.
There was some evidence that when aircraft were grounded, after the Sept 11 attack, day and night temperature differences increased. There has been talk that contrails made at night are causing significant warming - http://news.bbc.co.uk/1/hi/sci/tech/5082918.stm . I think that the evidence is becoming conclusive that contrails can change weather. Of course it took scientists 30 years to convice politicians that global warming was occurring so it will likely take a long time B4 contrail weather change becomes “scientific fact.” I predict airline &aircraft maker funded science institutes that will “disprove” this evidence in much the same way that special interest groups are currently disproving global warming and evelution.
This seems dubious to me. The sky (and the planet, or the United States) is a really, really, big thing, and airplanes aren’t that big. I would think that the portion of the atmosphere occupied by contrails would be almost infinitesimal. Maybe over a big city it would be significant, but still small. Roadways cause heat islands, but only around cities, and they occupy a tiny portion oof the globe I would imagine. With contrails there’s a whole third dimension that should make the percentage even smaller.
Well, Cecil made a reference to it in a recent column. You know, Cecil, the reason for being for this message board…
Dubious or not, it is so, as best we can tell. Contrails increase the reflectivity of that layer of the atmosphere measurably, as there is very little cloud there naturally. It’s not the percentage of the sky covered by contrails that matters, but the increase they cause from the natural extremely low amount.
As mentioned above the global temperature spike in the 3 days after 9/11 are about as conclusive evidence as there can possibly be in this kind of thing.
That was addressed in the link I posted.
7,700 square miles of coverage from 6 aircraft. Now do a quick search on the number of flights over the US in a day and the numbers are not so infinitesimal.