Contrails result from the interactions between the hot, humid air exhaust and outside air at low vapor pressure and low temperature; air at low temperature holds less water vapor. Vapor pressure is the pressure exerted by water vapor.
Saturation vapor pressure is a function of T. As the air temperature increases more water vapor can be held, up to as certain limit. For your purposes, an observation of how long the contrail lasts may be of some use. If the atmosphere is dry, then the contrail will dissipate very rapidly, but if atmosphere is close to saturation, then they will last far longer. However, as you point out, MungoP, you have a good grasp of this. When contrails are longer and present longer, the higher humidity may indicate higher temperatures, higher relative humidity (ratio of how much water vapor the air is holding and how much it can hold) and there is a higher potential for rain and potential condensation effects upon the jet engine performance. (Of course all the usual talk about air density, density altitude, apply as a sidebar).
Contrails will be better sustained at higher exhaust temperatures, in conjunction with cooler outside air, however, a more humid atmosphere will also contribute to longer contrail duration.
Contrails can dissipate vertically or spread horizontally into a thin cirrus layer. Increased jet aircraft traffic may contribute to more cloud cover, thus changing a given region's cloud cover. But, more to consider is the long atmospheric fate of trace gases like CO2 which is a positive forcing to water vapor (both are radiative active gases).
Last edited by jcbmack; 24th Sep 2009 at 23:00.