www.chicagotribune.com/news/asktomwhy/ct-wea-0216-asktom-20120216,0,4455225.story

Chicago Weather Center

Ask Tom why: Why are jet exhaust condensation trails more numerous on some days than others?

February 16, 2012

Advertisement

Dear Tom,

I am an airline pilot with 30 years experience. Why are jet exhaust condensation trails more numerous on some days than others?

—John Briggs, Glen Ellyn

Dear John,

Condensation trails, or contrails, are the delicate white cloud streamers frequently seen in clear air behind high-flying jet aircraft. They form when water vapor in jet engine exhaust gases sublimes (condenses) into visible ice crystals. Among the factors that affect contrail formation are humidity and availability of condensation nuclei in the air at flight level. Humid air (relative humidity greater than 95 percent) greatly enhances contrail density and duration. Contrails will evaporate almost instantaneously in dry air, or not appear at all. On occasion, the availability of condensation nuclei is minimal and exhaust fumes provide the necessary nuclei.