Interesting question, very interesting answers!
I reckon that the number of aircraft flying would make it more likely that there is an aircraft flying in the right atmospheric conditions to leave a con-trail. I may be completely wrong as I have never been to either Africa or Europe, but I'll take a shot. The exhaust from the jet contains a certain amount of water, it is held as vapour in the immediate jetstream but as we all know cold air can't hold as much water as hot air so as it enters the approx. -50C degree air it cools and if that air happens to be near saturation anyway the atmos says " no thankyou, can't take any more, out you go in the form of condensed water vapour(cloud)".
Maybe there is more humid air in Europe? Maybe it's just colder so can't hold the water as well, maybe neither but the number of a/c flying make the odds of a/c being in those conditions higher?
Would like others to comment as I am interested in con-trails purely because they look cool and I'd like to know if I am in the right ball-park.
I don't think there is an altitude below which they never form, if you cool the air quick enough and there is enough water in it a trail will form, at least that is what I assume after seeing trails coming off F-18 wings just at the point of rotation, the point where angle of attack is increased and therefor the speed of air over the wing is increased , lower temps etc.waffling now......might crank up the C182 and do some trials....
cjam....out.