What Causes Fogging on the Outside of the Windshield When Using Air Conditioning?

[StGabriel]

The fogging on the vehicle's windshield is caused by a significant temperature difference between the interior and exterior under current conditions. Solutions for windshield fogging: 1. Front windshield defrosting: Generally, the front windshield defrosting function is used to handle the front windshield based on the driving environment. If equipped with automatic air conditioning and the ability to select air vent positions, it is recommended to choose the front windshield and center vent positions and turn on the rear windshield heating function. This way, the temperature inside the car will be continuously regulated, preventing fogging. 2. Rear windshield heating: If it is not possible to select both directions for air venting simultaneously, turn on the front windshield defrosting function and the rear windshield heating function. If the side windows fog up, slightly opening the windows can quickly remove the fog. After closing the windows, once the interior temperature rises, no further fogging will occur.

[MacJourney]

With over 20 years of auto repair experience, I've seen this situation countless times. Simply put, it occurs when the surface temperature of the windshield drops too rapidly. During summer, blasting AC cold air directly at the glass causes the outer layer temperature to plummet, causing humid external air to condense into droplets upon contact. This phenomenon is particularly noticeable during sultry pre-storm weather or after car washes. The solution? Immediately switch off direct AC airflow, use medium-high fan speed with parallel airflow direction, and wait for the glass temperature to normalize - the fog will disappear. Avoid using wipers to scrape it off, as this will only smear the condensation further. For quick resolution, slightly open windows to balance interior/exterior temperatures, or use anti-fog spray as preventive maintenance.

[McMartin]

I remember one time when I was picking up my kid from school and it suddenly started pouring rain. As soon as I turned on the AC, the windshield became completely fogged up. The kindergarten teacher explained that it's the same principle as glasses fogging up—the cold air inside the car cools the glass to a temperature lower than the outside, causing the humid air to condense instantly upon hitting the cold glass. Later, I found that using the front window defogging mode works best. The key is to adjust the vents to blow parallel to the glass instead of blowing cold air directly at it. Additionally, warm air can also prevent fogging, though it might make the fog worse initially. If your kid in the back seat complains about the cold, it's safer to use the external air circulation with natural wind. Although it works slower, it won’t make them catch a cold.

[WalkerMarie]

The most annoying thing about driving in the south during the rainy season is the fogging up when the air conditioning is on. The excessive cooling from the AC makes the windows colder than the air temperature, causing condensation on the outside. Here are a few tested and effective methods: first, wipe off the surface fog with a towel; set the AC temperature no lower than 24°C; turn on the external air circulation with natural wind; and activate the rearview mirror heating function if available. If you're in a hurry and can't wait for natural defogging, briefly turn on the front windshield heating wires to assist. Don’t think fog on the outside is safe—it’s actually more dangerous than interior fogging, as it can instantly whiten your vision. Be extremely careful.

[GavinMarie]

The new interns in the company always complain about this. Actually, the misconception lies in using the MAX AC mode, which concentrates all the cold air on the windshield. It's recommended not to use the defog button (that's for solving internal fogging), but manually adjust to the chest mode and increase the temperature by two degrees. The key is to adjust the airflow direction to blow parallel to the glass, not perpendicularly impacting it. Treating the glass with a rain repellent spray when parked helps a lot, but avoid spraying it on the camera area. Nowadays, the smart defogging systems in new cars can detect temperature differences and adjust automatically, while older cars rely on experienced handling.