Not all vapor trails are high altitude or cold weather... On a day with high enough humidity condensation trails can show up just above sea level. All it takes is a pressure differential big enough over the surface of aircraft, usually seen in the turbulent areas at the wingtips. (Mostly seen when an aircraft is going very fast or pulling a hard manuever.) Low altitude vapor trails usually have nothing to do with engine exhaust and is just ambient moisture being condensed from the air. The noticable difference is the low altitude ones usually dissappate almost as soon as they are formed and wouldn't trail very far from the aircraft. The high altitude thing only applies to vapor trails that stay long after the aircraft has departed. In which case the jet exhaust explanation applies. BTW, the exhaust trail I suggested earlier isn't a condensation trail. Just some soot particles. You cold easily postwork it and make the sky just a tint darker where it would follow the aircraft. If you look at some photos you'll see what I mean.

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