Why does the windshield fog up when using air conditioning in summer?

[VanCarmen]

Reasons for windshield fogging when using air conditioning in summer: The outside temperature is lower than the inside temperature, causing water vapor inside the car to form many tiny water droplets. Since the surface of the windshield is not perfectly smooth (it has many small pits), these droplets adhere to the windshield, resulting in fogging. Here are two solutions: 1. Lower the temperature: Turn on the air conditioning to reduce the interior temperature to a certain level. 2. Use detergent: Apply a small amount of detergent to a cloth and wipe the inside surface of the windshield. This will prevent fogging even without air conditioning, as the detergent fills the pits on the glass surface, preventing water droplets from adhering. This method is highly effective in practice.

[Poppy]

I've been driving for over a decade, and foggy windows from AC use in summer are all too common—mostly caused by the big temperature difference inside and out. When the car's AC is blasting cold air, chilling the windows, and humid outside air hits them, condensation forms. Here are some practical tips: First, switch the AC to fresh air mode to vent out the moist air, crank the fan to max and aim it at the windshield—usually clears the fog in two minutes. Never forget to check the cabin air filter; a clogged, dusty one hampers dehumidifying. After parking, crack the windows slightly to let moisture escape and reduce buildup. Oh, and wiping the inside of windows with a diluted dish soap solution leaves a protective film that fights fog—works like a charm, tried and tested. If fog suddenly appears while driving, stay calm—lowering the AC temp a bit does the trick.

[Hugh]

As a long-haul driver who frequently travels long distances, the most dreaded situation in summer is fogging on the windshield obstructing visibility. The main reason is the high humidity inside the car, combined with the cold air conditioning lowering the glass temperature too much, causing water vapor to condense on it. My experience is to open the windows for two minutes before driving to ventilate the cabin, allowing the interior humidity to match the outside before closing the windows and turning on the AC. If fogging occurs, immediately press the defogger button—the one with the fan-shaped icon is specifically designed for this purpose. Simultaneously, switch to external circulation mode and set the fan speed to maximum, directing the airflow straight at the windshield. If there are four or five passengers in the car, it's advisable not to set the temperature below 22°C, as a smaller temperature difference reduces the likelihood of recurring fogging. Remember to check the AC drain pipe every three months for blockages, as poor drainage can lead to increased interior humidity. For long trips, keep a dry towel handy—if fogging becomes severe, you can safely pull over and quickly wipe the windshield.

[McVincent]

Last time I encountered this issue during a road trip with my kids, so I specifically consulted a car mechanic. The fogging caused by using the air conditioner in summer is essentially a physical phenomenon: humid hot air condenses into water when it meets cold glass. The solution involves three steps: First, press the defogging button on the center console; second, adjust the air vents to face the windshield; third, turn on the external air circulation to introduce dry air. Remember to also press the AC cooling button for optimal dehumidification. If it's a heavy rainy day with extremely high humidity, you can set the temperature to the lowest and direct the airflow toward the windshield for the best effect. A tip to prevent fogging: when parking, tilt the sunroof slightly to allow ventilation and avoid humidity buildup inside the car. New cars generally don’t have this issue for the first two years, but for cars older than five years, watch out for aging seals that may let moisture seep in.