A small difference between temperature and dew point. If temparature is say 4 celsius and dew point is 2 then that could be ideal because when a fast moving aircraft moves through the air, it compresses air and as air compresses, it cools down and if temperature and dewpoint is the same, you will have visible moisture as in clouds and contrails. the dewpoint is the temperature at which moist air saturates and condenses which in turn becomes clouds and contrails. This also explains why you get that cloud ring when an aircraft goes Mach 1.
contrails may or may not appear depending on the current weather conditions; contrails' altitude range changes every day.
When you're planning a visual interception and dogfight mission, the military weather service will tell you where to expect contrails, so that you don't fly at those altitude ranges and keep visually discreet.
I was just thinking about this the other day. I was near Weston-super-Mare in the UK and was watching a few planes fly over at high altitude. Both had contrails right up to the point where they crossed over to land (instead of the sea) where they abruptly stopped. It was most interesting to see!
Let's see if I can use some of my meteorology background to explain.
First a definition:
Contrails - Long, narrow, ice-crystal clouds that form behind jet planes flying at high altitudes in below-freezing temperatures
Contrails are generally found at high levels (where the temperature is roughly -25c/-32F about FL250) in nearly saturated conditions. Aircraft inflight emit water droplets into the atmosphere (from the exhaust) which then freeze in this cold / moist atmosphere. That's how you get 'em.
I remember a study being done during the period when no a/c were flying after 9/11 and the results stated that average daily variations in temperature (between high and low temps) increase by 2F.