Tuesday, February 26, 2013

Contrails over Portugal and Spain

Contrails are condensation trails that form behind high-altitude aircraft. The composition of contrails is practically identical to naturally-forming cirrus clouds; naturally high levels of humidity cause the clouds to form, and contrails form when airplanes inject extra water vapour into the atmosphere through their exhaust. Air temperatures must be -39°C (-38°F) or below in order for contrails to develop.

Depending on the humidity of the air, contrails can last seconds or many hours. If the air is dry, the contrails linger in the air for seconds to a few minutes. When the air is humid, the contrails can spread outward until they are difficult to distinguish from naturally occurring cirrus clouds. Though most of the contrails in humid air last a few hours, satellites have observed clusters of contrails lasting up to 14 hours and traveling for thousands of kilometres before dissipating.

Contrails do have an impact on climate. The long-lived and spreading contrails (like the ones pictured) reflect sunlight and trap infrared radiation. Even one contrail in a clear sky reduced the amount of radiation that reaches the Earth’s surface and at the same time increases the amount of infrared radiation absorbed by the atmosphere. Until now, it has been difficult to ascertain what overall impact these two effects have on climate.

Scientists at NASA’s Langley Research Center have developed a computer algorithm that searches through data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and distinguishes between natural cirrus clouds and young- to medium-aged contrails. This allowed the scientists to estimate how much contrails contribute to overall cirrus and cloud coverage.

The group published their findings in a 2013 article in Geophysical Research Letters (http://bit.ly/ZuGmdO). The group estimated that contrails cover between 0.07% and 0.40% of the Northern Hemisphere sky in a given year. When scaled to the Southern Hemisphere, the global mean coverage would be 0.07%. Coverage is greatest during winter and least during the summer. The researchers also concluded that contrails produce a slight net warming effect on the Earth. The researchers still have the challenge of detecting the older, wider contrails, like the ones in the image shown, to better estimate their coverage and impact on climate.

This image was taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite on February 15, 2013; it shows many contrails over Portugal and Spain.

Spangenberg, D. (2013, Feb. 11). Contrail radiative forcing over the Northern Hemisphere from 2006 Aqua MODIS data. Geophysical Research Letters. (http://bit.ly/ZuGmdO)
NASA image by Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response.

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