One of the more common suggestions when it comes to defogging a windshield is a simple one: Turn on the Recirculate feature for your car’s fan system. The reasoning behind this particular option is that it’s supposed to keep the interior air circulating and reduce moisture buildup on your windshield (since there are fans positioned just below it, on the dashboard). However, in colder temperatures it can have the opposite effect.
The biggest factor is less the temperature itself and more the amount of moisture in the air — both inside and outside of your car. If the air inside your car isn’t dry enough, recirculating it will just keep that moisture trapped and lead to a buildup of condensation. This, of course, will lead to fog. But chilly winter air is usually dry, so by keeping Recirculate turned off the fan system will pull in the less watery air from outside and keep all that moisture from building up on the glass.