Contrails—those cloudy tracks laid across the sky by jet planes—have a noteworthy effect on the local climate below, new research shows. A study recently published in the International Journal of Climatology suggests that in places where jets and their contrails are common, meteorologists could routinely add their effects to weather forecasts.
Contrails are more than just a pretty sky feature or aerial annoyance. Scientists saw a dramatic example of their importance in September 2001, when the U.S. government grounded commercial airlines for three days in the wake of the 9-11 terrorist attack. During that time, areas that were prone to contrails, like the Midwest, had significantly warmer days and cooler nights.
The study’s authors, Jase Bernhardt and Andrew Carleton, are two atmospheric researchers at Penn State University who made a more systematic effort to study the effects of contrails on daily temperatures. They learned that contrail “outbreaks” have definite effects on ground temperatures, and those effects may be predictable.
The researchers looked for contrails in satellite images, examining regions of the U.S. where contrails are common. To avoid the effects of snow and other influences, they picked the South in January 2008 and 2009, and the Midwest in April 2008 and 2009.
They found satellite images from clear days in which multiple contrails appeared, selecting outbreaks that lasted more than four hours. For each outbreak, they looked at the records from a weather station beneath the contrails and another station less than 100 miles away without contrails. For instance, they paired weather records on April 2, 2009 from Erie, Pennsylvania (contrails) and Buffalo, New York (no contrails).
The results were clear. By warming the nights and cooling the days, contrails reduced the daily temperature range, on average, by 6.4 degrees Fahrenheit in the South in January, and 5.3 degrees in the Midwest in April. Individual cases varied, though, from no effect to as much as 27 degrees. The effect on daytime highs was greater than the effect on nighttime lows.
In causing warmer nights and cooler days, contrails act like ordinary clouds. We see the same effect in the Bay Area whenever a blanket of “marine layer” clouds covers the countryside.
If these results hold up in further studies and in other regions, they may lead to more accurate predictions of daytime highs and nighttime lows in areas of high jet traffic.