What in the World?

Most of the white contrails you see in the sky form when hot air from an airplane’s engine mixes with cold air in the atmosphere. That causes water in the plane’s exhaust to condense, or change from a gas into a liquid. Contrails commonly form behind airplanes flying at altitudes between 9,750 and 12,800 meters (32,000 and 42,000 feet). These vapor trails can last from just a few seconds to hours, depending on the humidity. If moisture levels in the air are low, contrails are short-lived. Higher humidity can lead to longer-lasting contrails. The ones shown here were made by jets flying over Utah.

The rainbow effect seen by Mukherjee doesn’t happen often. And unlike common white contrails, colorful ones aren’t created by an airplane’s exhaust. They instead form from air moving over the plane’s wings. If the air is moving fast enough, its air pressure—the force of air pressing against the surface of an object—will drop. As the pressure of the air drops, so does its temperature. That causes water droplets of all different sizes in the air to quickly freeze. Because the bits of ice are different sizes, as sunlight shines through them, they refract, or bend, the light differently. This creates a range of iridescent colors behind the plane.