Credit: Carol Clark (left), Kristen Meymaris (center), Susan Gallagher (right)
The amount of moisture present in the atmosphere determines what type of contrail ;can form. A short-lived contrail (left) forms immediately behind an airplane as a bright white line that lasts for only a short while if the air is somewhat moist (as compared to air that is very moist). A persistent non-spreading contrail (center) forms when the air is very moist and will stay in the sky long after the airplane has flown out of sight. It can last for a few minutes or longer than a day, and it keeps its shape of a thin line. A persistent spreading contrail (right) also forms in very moist air but spreads out across the sky. It grows wider and fuzzier as time passes. Sometimes contrails will actually take on the characteristics of a natural cirrus cloud and no longer look like contrails, thus becoming human-made clouds.