Fogged up, “unclear” windows are not just an inconvenience, they present a “clear” driving danger.
When the air temperature inside a vehicle is warmer than the temperature outside, the water molecules inside the cabin can go through a process of condensation and collect on the windshield and windows, where the dueling temperatures collide. This causes a cloudy moisture to form on the windows, especially on cold or rainy days, inhibiting visibility. Defrosters help combat this phenomenon by clearing away unwanted moisture so that drivers can see out the windows and operate the vehicle safely.
On most late-model vehicles, the front defroster consists of vents and operates with the same blower motor and fan as the air conditioning system. The A/C (regardless of its set temperature) removes the moisture from the air inside the cabin through condensation and directs it out through the bottom of the vehicle. The system then pumps dry air into the car. When you turn on the defroster, the vents direct this dry air up toward the windshield to evaporate the moisture. The defroster also equalizes the temperature of the glass with the air temperatures.
Unfortunately, this means if your air conditioner isn’t working properly, your front defroster will also not work well, so checking and maintaining your A/C system is about more than just comfort, it’s about safety as well.