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.
·
References
· Duda, D. (2013, Feb. 11). Estimation
of 2006 Northern Hemisphere contrail coverage using MODIS data. Geophysical Research Letters.
· Spangenberg, D. (2013, Feb. 11). Contrail radiative forcing over the Northern Hemisphere from
2006 Aqua MODIS data.Geophysical Research Letters.
NASA image by Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response. Caption by Adam Voiland, with information from Patrick Minnis
and Douglas Spangenberg.
Instrument:
Terra -
MODIS
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