What does a contrail weigh?
I find that simple arithmetic is often informative in examining some of these sort of theories, to make an assessment of plausibility. How much vapour is there, by weight, in a contrail?
I just looked out of the window and saw a couple of airliners pulling contrails. Typically they will produce a pair of trails, which look roughly cylindrical and extend slightly beyond the wingtips. Assuming a wingspan of a little under 40 metres (about right for a 757, say) we can estimate that the radius of each cylinder is 10 metres, so the area is about 300 square metres (truncating pi to a single digit; this is order of magnitude stuff).
Now length. Let's take the example of an aircraft flying at a height of 10,000 metres (33,000 feet), passing overhead, and producing a contrail from horizon to horizon - not an unusual scenario, but about the maximum contrail length that can be seen by a single observer. Wikipedia gives a handy formula for estimating the distance from a point at altitude to the horizon; multiply the height in metres by 13, then take the square root, and the result is the distance in kilometres. When the airliner crosses my horizon, the point where I'm standing is at its horizon, so that's how far away it is. Multiply by two - it goes from one horizon to the other - and we get a total length of 690 kilometres (or about 430 miles). Multiply this by the area, and we have a contrail volume of about 200 million cubic metres.
Estimating the water content of a cloud is a very complicated business about which I know very little. However, let's make some estimates. The visibility of contrails tends to be very good, so optically they can be taken to behave something like cumulus clouds. A quick Google gives:
http://books.google.com/books?id=hv...ts=ajA-8jOQ2G&sig=afGWGOEF5AoW6NY2lSwWrRyyxLQ
Cumulus clouds vary in water content from 0.2 to 1.0 grammes per cubic metre. Let's take 0.5 as a sensible mid-range number.
That gives us a total weight of water in the contrail as about 100 million grammes, or
100 metric tons. And that's just the length of contrail we can see from a single point of observation - 430 miles is a pretty short distance for an airline flight. Just for comparison, the maximum payload of a C-17 Globemaster is 77.5 metric tons.
For me, that more or less proves that the source of contrail clouds must be atmospheric. There simply isn't enough payload capacity, even in the biggest airliners, to release that much of anything.
Dave