Depending on the temperature and the situation you’ll want to attack the condensation on your windshield and windows with different methods.
In chilly weather, you’re likely worried about the interior of the glass fogging up. The fogging occurs because of the difference in temperature and humidity between the inside of the car and the outside of the car. Outside it’s cold and but still generally dry because the air in winter has very low humidity, and inside it’s warmer and more humid thanks to your body heat and the moisture from your breath being trapped inside.
When you need the window defogged right away, the fastest way to do so is to rapidly lower the inside temperature so that the moisture stops condensing on the glass. Turning the defrost vent on without heat or opening the windows in cold weather is the fastest way to dissipate the fog on the window. While fast, it isn’t very comfortable for the occupants of the car so it’s best reserved for times where you find the window suddenly fogging up and you need to see out in a hurry.
It’s significantly more comfortable to use your heater and use other methods to manage the moisture than it is to keep your car freezing cold. Warming the inside of the car and blowing warm air across the windshield using the defrost setting will evaporate the moisture. Skip using the recirculate feature if your car has it, you want to push the humid air out of the car and bring dry air in from the outside.
In particularly humid weather (a typical North Carolina summer) you have to deal with the opposite problem. High humidity and temperature on the outside of the car and low humidity and temperature on the inside of the car make it necessary to reverse your technique. You can use your wipers to constantly clean away the fog from the exterior but that’s a bit of a hassle. You can turn the AC down or off entirely, or you can turn the heat on low and warm the inside of the glass to stop the moisture from condensing on the outside. Fog on the outside of the windshield or windows is a less common than fogging on the inside, but when it happens it can be a real pain.
We highly recommend that you keep your windows extra clean. Not only do clean windows help with visibility even when you’re not dealing with fog, clean windows are less prone to fogging.
There are further steps you can take, such as using anti-fog glass treatments from brands like Aquapel and Rain-X. These products are easily applied to the inside surface of your windshield or window glass and result in less condensation and fogging for improved visibility.