The condensation trails that form behind high-altitude aircraft, or contrails, are one of the most visible signs of the human impact on the atmosphere. On February 15, 2013, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this view of numerous contrails over Portugal and Spain.
The composition of these long, narrow clouds is virtually identical to naturally-forming cirrus clouds. However, while naturally high levels of humidity cause the clouds, contrails form when airplanes inject extra water vapor into the atmosphere through their exhaust. In order for contrails to develop, air temperatures must be -39°C (-38°F) or below.
The humidity of the air affects how long contrails last. When air is dry, contrails last just seconds or minutes. But when the air is humid, contrails can be long-lived and spread outward until they become difficult to distinguish from naturally occurring cirrus clouds. Satellites have observed clusters of contrails lasting as long as 14 hours and traveling for thousands of kilometers before dissipating; however, most remain visible for only a few hours.
Contrails have an impact on climate. Long-lived, spreading contrails like the ones shown here are of particular interest to researchers because they reflect sunlight and trap infrared radiation. A contrail in an otherwise clear sky reduces the amount of solar radiation that reaches Earth’s surface, while increasing the amount of infrared radiation absorbed by the atmosphere. These opposing effects make it difficult to sort out the overall impact on climate.
However, a group of scientists at NASA’s Langley Research Center have made progress. They have developed a computer algorithm that searches through data from MODIS and automatically distinguishes between natural cirrus clouds and young- to medium-aged contrails. This has made it easier to estimate how much contrails contribute to overall cirrus and cloud coverage. In a study published in 2013, the group estimated that contrails cover between 0.07 percent to 0.40 percent of the sky in a given year. They also concluded that contrails produce a slight net warming effect on the Earth.
There are still problems the researchers are working to solve. “Detecting the older, wider contrails, like many of those in this MODIS image, remains a challenge and we are still unable to estimate their coverage and impact on climate as well as we would like,” noted Patrick Minnis, a NASA Langley scientist.
When viewed from space, clusters of airplane contrails make distinctive geometrical patterns. Scientists are using satellite detections of contrails to sort out how the long, narrow cirrus clouds impact our climate system.