Foggy windows can obscure your vision when driving. Why? Blame it on air temperature. Add in dew point, then calculate the relative humidity. That equals condensation or the change of gas to liquid in the atmosphere, better known as fog.
Car windows fog up when there is a lot of moisture in the air and every time you and your passengers exhale you add more moisture to the air in your car. The more people in your car, the more likely your windows will fog up.
You can control the problem by keeping a steady supply of air moving across your windows. First, turn on the defroster to high and try cracking the window. You can also use the fresh air or auto fan setting. Some cars even have a selector for outside air.
If that doesn't clear the windows, use the air conditioner. The air conditioner dries out the air and removes moisture. Another way to prevent the buildup of fog is to check your vents. Make sure they are open so air can exit even if the windows and doors are closed. Vents can also get clogged with dust and debris so make sure they're clean so air can leave the car along with all the moisture.
Foggy windows can be a real safety hazard. Don't drive if you can't see out the window.