Flykal From Australia, joined Sep 2003, 435 posts, RR: 5 Reply 1, posted (4 years 2 months 1 week 5 days 19 hours ago) and read 517 times:
You are correct when you say that many factors affect the formation of contrails, but primarily it depends on atmospheric humidity. There is no guarantee that an aircraft at FL390 will generate contrails.
Certain regions will change seasonally so a certain times of the year you may see contrails, while other seasons may not produce them. Obviously humidity is dependant on the amount of water in the air and contrails are also dependant on the temperature of the air and the heat and water that comes out of the jet engine exhaust.
Higher humidity will generally mean that contrails stick around for longer, while lower humidity means you get contrails with a shorter life span.
One doesn't discover new lands without consenting to lose sight of the shore for a very long time