A couple of weekends ago, when the weather was clear and near 90 degrees and so many of us were outside trying to soak up as much warm sunshine as we could, a lot of people noticed that jet trails (contrails) were crisscrossed across the sky all day long.
The reason for the long-lasting contrails is very simple and completely atmospheric. That weekend, at altitudes above 30,000 feet, humidity levels were high and winds were very light, resulting in conditions which allowed contrails to form and linger. That far up in the sky, the temperature is perpetually well below zero and the air is usually very dry. So jet exhaust tends to disperse quickly.
But when conditions are just right, the moisture in the jet exhaust forms a cloud which will remain in place if there is not much wind. We get the same effect sometimes on cold winter days when our own breath forms a lingering fog around our head.