Re: Is there any solid evidence for 'chemtrails'?

What makes a contrail visible is water (or ice, at that high altitude).  
Airplane exhaust is very similar to car exhaust, and by weight the primary 
constituents of airplane exhaust are nitric oxide (NO) and carbon monoxide, 
although there will be traces of other products of partial combustion of 
jet fuel.  Generally, plane exhausts are a bit cleaner than car exhausts.  
By the time any pollutant from airplane exhaust is down where people or 
birds can breathe it, it is extremely dilute, and may have been 
photo-oxidized to carbon dioxide, water, or some other oxide.  The worst 
pollutant is NO, which acts to scavenge oxygen from the ozone layer, 
thereby assisting in ozone layer destruction.  Nothing sinister.