Another study found a higher diurnal temperature range attributed to a decreased planetary albedo with the absence of them over North America for several days after September 11, 2001.  
Studies such as the AEAP and SUCCESS have found that these artificial entities have a slight positive radiative forcing effect, with one study indicating they could account for a warming trend in the US from 1975 to '94. Another study found a higher diurnal temperature range attributed to a decreased planetary albedo with the absence of them over North America for several days after September 11, 2001.  They persist longer in humid air before spreading out, and they can be formed from wingtip vortices but are usually the result of combustion.  For 10 points, name these high altitude cirrus clouds that form as water vapor condenses behind a high flying airplane.
Studies such as the AEAP and SUCCESS have found that these artificial entities have a slight positive radiative forcing effect, with one study indicating they could account for a warming trend in the US from 1975 to '94.
contrails [or condensation trails; or vapor trails; generously prompt on "clouds" before it's said]