Sunset

Sunlight illuminates the contrails of jet in the early evening sky above Chicago.

Depending on a plane’s altitude, and the temperature and humidity of the atmosphere, contrails may vary in their thickness, extent and duration. The nature and persistence of jet contrails can be used to predict the weather. A thin, short-lived contrail indicates low-humidity air at high altitude, a sign of fair weather, whereas a thick, long-lasting contrail reflects humid air at high altitudes and can be an early indicator of a storm.

Scientific American