Weather Whys from Texas A&M Geosciences Center

Q: You sometimes hear of contrails.  What are they?
A:  Contrails are what appear to be long lines of clouds high in the sky and they are formed from jet aircraft exhaust, explains Brent McRoberts of Texas A&M University. “Contrails are created when very cold air freezes the warm and humid engine exhaust,” he says.  “Air temperatures where contrails are formed are usually minus 40 degrees or colder. Contrails could be compared to exhaling in winter and seeing your breath because it’s so cold. When water vapor from the exhaust hits the cold air, ice crystals develop and they produce a contrail.”
Q: Do contrails affect the weather?
A: This is a question that is beginning to be seriously debated worldwide, McRoberts adds.  “More and more scientists believe contrails act like natural cirrus clouds and block out solar energy from above and keep in the heat from below,” he adds.  “A group of NASA scientists estimated that contrails cover about 0.1 percent to 0.4 percent of the Earth’s skies in an average year. In heavy air traffic areas, as much as 20 percent of the sky may be covered by contrails.  Another study shows that lower atmospheric temperatures across North America rose almost 0.5 degree per decade from 1975 to 1994, which is why some researchers believe contrails may have an impact on climate change over long periods of time.”