Why does the outside of the windshield fog up when driving in the rain?
2 Answers
The reason the outside of the windshield fogs up when driving in the rain is due to a significant temperature difference or humidity variation between the indoor and outdoor environments. If your vehicle is equipped with a manual air conditioning system, turn on the front window defogging mode, set the fan speed to maximum, switch on the A/C, and adjust the temperature to the lowest setting to dry the fog on the windshield. If your vehicle has an automatic air conditioning system, usually pressing the one-touch defrost button will quickly remove the fog. If the ambient temperature is too cold for using the air conditioner, you can also turn on the heater, set the airflow to maximum, and activate the front window defogging mode. The fog should disappear within minutes. Most rear windshields come with electric heating—simply turn on the rear windshield and mirror heating functions to quickly remove fog and prevent it from forming again.
Fogging on the outside of the windshield during rainy driving is primarily a natural phenomenon caused by temperature differences. Rainwater rapidly cools the exterior glass, and when the moisture in the car's air contacts the cold glass, it condenses into a foggy film. This situation is common in humid weather as it amplifies the effects of moisture. I often encounter this issue during long-distance driving and have found that promptly turning on the wipers can temporarily clear the fog. However, the fundamental solution is to adjust the car's air conditioning to the external circulation mode, preventing warm air inside the car from hitting the cold exterior glass. Additionally, low visibility during rain is inherently dangerous, and if the fogging is severe, safety comes first—pull over to handle it. For prevention, I always check the wiper blades and clean the glass surface after each car wash to ensure minimal oil film, reducing the chance of condensation.
