Contrails are one of those aviation/weather phenomena that are so common we take them for granted. Ever wondered how they are formed? Why do some contrails look different from others? Read on for a short, simplified-for-dummies lesson on contrail science!
The term “Contrail” is a blend of “Condensation Trail.” You’ve seen condensation; sure you have! It’s the reason your mom yelled at you for not using a coaster under a cold can of soda. Moisture from the warm air around the cold drink condenses and forms drops of water on the outside of the can. This same process also generates clouds and fog. Moist air + a drop in temperature = condensation. Clouds are really the same as the droplets of water on your soda can; except the droplets in a cloud are much, much smaller (about 10 microns or 1/100 mm).
Let’s create some man-made fog!
Airplanes are wonderful condensation generators; they create two different types using the same physics.
1. Engine Exhaust Condensation
Gas powered engines, like those that propel cars and trucks, produce a hot exhaust composed of mostly carbon dioxide and water vapor. You’ve probably seen water dripping from the exhausts of cars at traffic lights; this is condensed water vapor. Airplanes are no different. Jets push a lot of hot air and water vapor out their exhausts. When flying at high altitude, the air is very cold. When the cold, moist air combines with the warm engine exhaust, the moisture quickly condenses into tiny water droplets or frozen crystals and a contrail is born. That’s all there is to it! The most common contrails in the sky are created in this manner. Louisville, KY meteorologist Ryan Hoke created a nice info-graphic for exhaust contrails (right).
Here are two examples of exhaust contrails. Both are Boeing 747s.
Watch an exhaust contrail being generated up-close:
2. Aerodynamic Condensation
I find aerodynamic contrails really fascinating. They’re not as common as exhaust contrails, but if you keep your eyes open, you’ll see them. An airplane wing generates lift by creating a difference in pressure between its upper and lower surfaces. The curvature of the upper wing causes the airflow to accelerate thus lowing the pressure of the air above the wing (Bernoulli’s Principle). As the air pressure over the wing decreases, the temperature of this air also decreases (Combined gas law). When the wing is generating lift in humid air, we have the ingredients needed to form condensation: moist air + a drop in temperature = condensation.
The best place to see aerodynamic condensation is looking out an airliner window on takeoff or landing when there is really high humidity, like on a foggy or rainy morning. The condensation looks like a fog that forms above the rear half of the wing. Sometimes it appears at the wingtips or edges of the flaps as a tightly wound, horizontal spiral. Aerodynamic condensation occasionally occurs at high altitudes producing some spectacular results.
This video, shot in Taipei, Taiwan, has some excellent examples of aerodynamic condensation. Due to its moist, tropical climate, Taipei is a perfect place to find this phenomenon.
You’ll immediately recognize them by the sheet-like cloud trailing the aircraft. Often, there will be an iridescent or rainbow effect associated with the contrail as the sunlight diffracts through the ice crystals.
Another place to see some cool aerodynamic condensation is an air show! Military fighter jets often generate impressive condensation due to their high speeds and extreme maneuvers.
Every once in a while, you’ll get treated to both types of contrails at the same time. In this photo, an Atlas Air 747 generates both exhaust condensation from the engines and aerodynamic condensation from the horizontal stabilizer (tail wing). Extra bonus points anytime you see both types of contrails at the same time!
Distrails: The Anti-Contrail
Dissipation trails or distrails are a rare phenomenon. They are the opposite of contrails. Distrails look like a clear path cut through a thin cloud layer. They are formed two different ways:
1. When an aircraft flies through a thin cloud layer, the heat from the engines can turn ice crystals into vapor, leaving a clear path through the cloud.
2. As an aircraft flies through a cloud layer made of super-cooled water droplets (water that is below freezing, but not frozen), the aerodynamic disturbance causes the droplets to spontaneously freeze. The frozen droplets fall below the cloud deck and/or evaporate.
Take a look skyward on the next clear day and take in some contrails (or distrails)!