I DIDN’T KNOW THAT

Question: I read your recent explanation of contrails, and I don’t believe it. I have seen photos of contrails forming at a plane’s wingtips, not behind the motors.

Answer: Condensation trails at the wingtips are a separate, but related, issue, according to Jenn Stroud Rossmann of Harvey Mudd College. Under conditions of high humidity, condensation can occur anywhere there is a vortex, such as at the tips of wings. The high-speed flow of the air causes an area of low pressure to form, and that in turn causes water vapor to condense into visible form. But condensation trails from the engines contain much larger quantities of water vapor emitted by the engines themselves, and thus are more visible and persist longer. Such condensation can occur whenever there is high humidity and an area of low pressure, Rossmann notes. One can occasionally see little clouds of condensation over the wings, where the pressure is lower.