"This digital photograph taken through the windows of the International Space Station on May 15, 2002, shows condensation trails over the Rhône Valley in the region west of Lyon. Condensation trails—or contrails—are straight lines of ice crystals that form in the wake of jet liners where air temperatures at altitude are lower than about ?40°C. Newer contrails are thin whereas older trails have widened with time as a result of light winds. Because of this tendency for thin contrails to cover greater areas with time, it is estimated that these “artificial clouds” cover 0.1% of the planet's surface. Percentages are far higher in some places such as southern California, the Ohio River Valley and parts of Europe, as illustrated here. The climatic impact of such clouds is poorly understood. In an unprecedented experiment following the September 11 grounding of all aircraft in the United States, researchers reported in the August 8 issue of Nature that temperatures in areas usually affected by contrail blankets fluctuated higher and lower during the contrail-free period. Despite this dramatic conclusion, it is nevertheless too early to know whether contrails produce a net warming or cooling of the atmosphere. Whereas cirrus clouds seem to have a net warming effect, contrails are denser and thus may produce the opposite effect."
http://visibleearth.nasa.gov/view_rec.php?id=3450
"Numerous airplane contrails crisscross the English Channel, providing visual proof of how common air travel is. Contrails form when hot, humid jet exhaust is expelled into the cold air at high altitudes, typically more than 8 kilometers (5 miles), above the ground. The exhaust freezes directly into ice crystals, forming thin streaks along the path of the jet. The resulting condensation trails only last a few hours, as can be seen in this Moderate Resolution Imaging Spectroradiometer (MODIS) image pair. Many of the contrails that were visible in the morning overpass have faded in the afternoon overpass. This image pair, acquired on December 9, 2003, by the Aqua and the Terra satellites, also shows the increase in air traffic during the day. Many more contrails are visible in the afternoon compared to the morning."
http://visibleearth.nasa.gov/view_rec.php?id=6143
"While modern air travel is a necessity to modern life, its effect on the planet’s radiation budget—the balance between the planet’s incoming sunlight and outgoing heat energy, which drives climate change—is not well understood. This true-color Moderate Resolution Imaging Spectroradiometer (MODIS) image, acquired on April 25, 2004 by the Terra satellite, shows a web of contrails over northwestern Europe. These contrails are straight lines of ice crystals that form in the wake of jet liners where air temperatures at altitude are lower than about 40°C. Newer contrails are thin whereas older trails have widened with time as a result of light winds. The problem with contrails is that they can spread into extensive high, thin cirrus clouds, which tend to warm the Earth because they reflect less sunlight back to space than the amount of heat they trap. Because of this tendency for thin contrails to cover greater areas with time, it is estimated that these artificial clouds cover 0.1% of the planet’s surface. Percentages are far higher in places with more air traffic, such as southern California, the Ohio River Valley and parts of Europe, as illustrated here. Tracking the formation of contrails is key to determining their contribution to cirrus clouds and their effect on the energy balance. Information and studies about contrails can be found at the Earth Observatory."
http://visibleearth.nasa.gov/view_rec.php?id=6575
"This SeaWiFS view of Lake Superior and Lake Michigan took in several contrails and their shadows. Contrails have recently been cited as having potentially significant impacts on global climate."
http://visibleearth.nasa.gov/view_rec.php?id=1364
"These odd cloud formation over the Pacific Ocean southwest of California possibly started out as a collection of airplane contrails."
http://visibleearth.nasa.gov/view_rec.php?id=984
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