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.