Comment: So the two responses I got

(See in situ)

In reply to comment: 1940's air planes spew smoke... (see in situ)

So the two responses I got

One says that 1940's planes spewed smoke, conveniently ignoring the photographic evidence from the 50's, 60's, 70's, and 80's, and the other suggests that it's all optical illusions.

But even the claim about the 1940's is wrong. Burning fossil fuels produces water vapor. Depending on the temperature and humidity, the water vapor may or may not condense. If it does condense, then at high altitude it will instantly freeze. Again depending on temperature and humidity the ice may sublimate quickly, or slowly, or with high enough humidity the ice crystals may even grow. See this article:
http://contrailscience.com/contrails-are-condensation-but-no...

Watch this:
http://contrailscience.com/memphis-belle-wwii-bomber-contrai...
and pay attention to how they explained contrails in the 1940's. It's condensation, he says it right there in this 1940's video. You don't get condensation from smoke, you get it from water vapor.

So to recap, contrails have been around since long before most people reading this were even born, and scientists explained contrails seventy years ago the same way they explain them now. Photographic and video evidence confirms that contrails have always behaved the way they do now, sometimes dispersing quickly, sometimes spreading out and lingering, etc. The scientific principles of water vapor production from burning fossil fuels, condensation depending on temperature and humidity, water freezing at low temperatures, sublimation, etc., are all very, very basic and well-understood principles.