How do contrails form?
A contrail is the condensation trail that is left behind by a passing jet plane. Jet fuel is made of carbon and hydrogen, and when it burns, it combines with oxygen. So most of the exhaust is made of carbon dioxide and water. The water is usually an invisible vapor, unless the temperature, winds and humidity in the upper atmosphere are just right. Then white contrails form when the moisture in the exhaust condenses. It’s similar to being able to see your breath when you exhale on cold days. Cold air can’t hold as much moisture as warm air, so moisture condenses into a visible cloud. Temperatures at the high altitudes reached by jet planes are usually below -38 degrees Celsius. There are three kinds of contrails: short-lived, persistent, and persistent spreading. Short-lived contrails resemble short white lines behind the plane. They only last 30 minutes or so, because there is so little moisture in the air that it quickly evaporates. Persistent contrails are long white lines that are s
