From: Visible Earth

"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."