Just the other day the early morning dew fogged up many windshields. The reason for foggy windows has to do with temperature and the air's moisture content. On a cold day, any moisture turns to condensation when it hits air next to the windows that's below a certain temperature, called the dew point. When the temperature inside the car heats up a vapour barrier builds up and tries to escape and hits the windshield causing it to fog. A dirty windshield will fog up faster because there are dirt particles for the condensate to cling onto.
Many people don't take the time to defrost their vehicle. Crank down the window, turn on the defrost and air conditioner and on the road they go looking through a peek hole of clear windshield.