The only clouds in a clear cold sky today were contrails, the condensation trails left by aircraft exhaust gases turned to ice crystals at high altitudes. They're evidence of the pollution of our skies by aircraft. For three days after 9/11, all commercial flights over the US were grounded. The exhaust fumes containing water, carbon dioxide, oxides of sulphur and nitrogen, hydrocarbons, carbon monoxide, unburned fuel and particles of soot and metal were dispersed. The difference between daytime and night-time temperatures was greater than normal, showing that contrails, which can spread into sheets of persistent cirrus, have a warming blanket effect.
For another contrail picture, click on the thumbnail (right).