Agreed, outside air contributes to fogging. A/C systems do a wonderful job of drying the air, so recirculating and cooling only cabin air is the best defogger there is.
But, in the winter you want warm air, which works well also. On many mondern cars when you turn on the defrost setting, you get both: first the air is cooled by the a/c to help dry it, then it is heated before exiting the vents.
On my daily driver, I have to leave the a/c compressor switched on even in the winter or the windows almost instantly fog (HVAC in auto mode) in all but the dryest of days.
Last edited by DavidF; 11/02/2012 2:54 PM.
