No Trail, Short Trail, Lasting Trail: What Decides a Plane's Contrail?

Why some planes leave no contrail, some a short one that fades, and some a trail that spreads for hours: two rules in the air decide it, and the plane mostly decides which side of them it sits on. A contrail forms only when engine exhaust, mixing with very cold air, briefly reaches liquid-water saturation. It lasts only if the surrounding air is supersaturated with respect to ice, which happens in layers often only a few hundred meters thick. We follow a 1999 test over Germany in which an Airbus A340 and an old Boeing 707 climbed wing by wing and, for one stretch, only the more efficient Airbus drew trails. Then we look at why lasting trails matter for aviation's climate effect, and what happened when airline pilots tried to fly around the layers. Diagrams are simplified illustrations. The 70-flight avoidance results are as reported by the companies involved; the Japanese-airspace and lower-cruise figures are model results. #Contrails #Aviation #AtmosphericScience #HowItWorks