A windshield fogs up primarily due to condensation, which happens when warm, moist air inside the car meets the cold windshield glass. This temperature difference causes the moisture in the air to condense on the glass surface, creating a foggy layer that reduces visibility. Factors that contribute include high humidity inside the car (often from breath or damp items), a dirty windshield that attracts moisture, and temperature differences between inside and outside the vehicle.
To reduce fogging, it's effective to use the car's defrost setting which directs warm, dry air onto the windshield to evaporate the moisture. Turning on the air conditioning while using the defrost helps because the A/C removes moisture from the air. Avoiding the recirculation mode also helps as fresh, dry air is brought in. Keeping the inside windshield clean from dirt and oily residues can prevent moisture buildup, and some use products like anti-fog sprays for extra help.
In summary, windshield fogging is caused by condensation due to temperature differences and humidity inside the car. Managing humidity levels and temperature inside the vehicle with proper use of heating, A/C, and defrost settings can prevent or clear the fog effectively.
