Why does the car window fog up when the air conditioner is on in summer?

[MacAaron]

When the air conditioner is turned on inside the car during summer, the outside temperature is lower than the inside temperature, causing the water vapor inside the car to form many tiny water droplets. The surface of the windshield is not perfectly smooth and has many small indentations, so these tiny water droplets adhere to the windshield, which is the reason for the fogging on the glass. The air conditioning system can be used to remove the fog from the glass, whether in winter or summer. In winter, turning on the cold air conditioning can effectively remove the fog on the glass. The best method is to switch on the air recirculation mode to allow full air circulation, which speeds up the evaporation of the fog, quickly removing the condensation on the glass.

[StSawyer]

Fogging up of car windows when using the air conditioning in summer is actually caused by temperature differences. I often encounter this situation, such as when driving to the countryside with the AC turned up high, making the interior extremely cold while the outside is hot and humid. When the window glass gets cold, the moisture condenses into tiny water droplets. The reason is that the air conditioning rapidly lowers the interior temperature, causing the surface temperature of the windows to drop below the dew point, which naturally causes the moisture in the air to accumulate. It gets worse when humidity is high, such as on rainy days or after sweating inside the car. I recommend not setting the AC temperature too low—around 25°C is ideal—while switching to defog mode or turning on the external circulation to quickly blow away the fog. This ensures clearer visibility while driving and avoids safety risks. Regularly cleaning up clutter inside the car to reduce moisture sources can also help prevent fogging.

[Judah]

Fogging on car windows caused by air conditioning cooling is purely a physical phenomenon, which I always observe while driving. Specifically, the air conditioning cools the interior, lowering the temperature of the window glass. If the outside environment is humid and hot, moisture condenses upon contact with the cold glass. This is similar to frost forming on a refrigerator door. In summer, when humidity is high, the likelihood of fogging increases, especially after rain or near the coast. I recommend not just using the cold air setting when turning on the air conditioning but also adjusting the fan speed and direction appropriately, such as directing it at the windows for a few moments. Regularly checking if the air conditioning system is effectively dehumidifying can reduce fog buildup, making driving safer and more comfortable.

[MacGabriella]

After years of driving, I've found that turning on the AC causes window fogging due to the temperature difference between inside and outside. When it's chilly inside the car while the outside is warm and humid, water droplets form on the windows. The simplest solution is to crack open a window or use the fan to blow air onto the windshield. Keeping the interior dry daily and preventing moisture buildup significantly reduces the problem.

[GemmaMarie]

The main reason for fogging up windows when using the air conditioning is that the cold air cools the windows too quickly, causing moisture to condense into mist. From an energy-saving perspective, setting the AC too low not only causes fogging but also wastes electricity. It's recommended to set the temperature above 26 degrees and avoid blasting the fan continuously. Using the recirculation mode initially helps with faster dehumidification and quicker mist dissipation. This approach saves fuel and ensures safety, a tip I often share with friends.

[DePaisley]

When I first got my driver's license, I was curious about why the windows fog up when using the AC in summer. My instructor explained: the car's AC is cold, while it's hot and humid outside, causing moisture to condense on the glass. When using the AC, avoid blowing cold air directly at the windows; pressing the defrost button quickly solves the problem. It's recommended for beginners to use the AC moderately, setting the temperature around 26°C, not too low, to prevent fogging that could affect visibility. You'll get used to it over time.