Should You Use Cold or Warm Air to Defog the Windshield?
2 Answers
Both cold and warm air can be used to defog car windows, but warm air is slower than cold air. The most effective method is to use cold air, which blows dry cold air directly onto the glass, preventing water vapor from condensing on the surface and achieving defogging. The method of using warm air is similar to cold air defogging, but it's important to note that warm air should not be used on rainy days as it can initially worsen the fogging. When the car windows fog up, it's best to use the external circulation mode. The reason for car window fogging: There is a temperature difference between the inside and outside of the car. If the temperatures inside and outside the car are different, the side with the lower temperature will have a lower saturated vapor pressure on its surface compared to the surrounding environment, causing water vapor to gather on the glass surface and form tiny droplets, resulting in fogging. Methods to defog car windows: Air Conditioning Cooling: Use the air conditioning's cooling and dehumidifying function to reduce air humidity and remove fog. In summer, especially when multiple people enter the car and the air conditioning is not turned on promptly, the exhaled air has high humidity, quickly causing the windshield to fog up. In this case, turn on the air conditioning to blow cold air onto the windshield, utilizing the dehumidifying function to remove the fog quickly. If the humidity is too high and the air conditioning is ineffective, slightly open the windows to rapidly reduce the humidity inside the cabin. Combining this with the air conditioning can speed up the process. Air Conditioning Warm Air: Use the method of reducing the temperature difference to remove fog. In winter, blowing warm air onto the glass quickly raises the temperature of the windshield, reducing the temperature difference between the inner and outer surfaces of the glass, thereby preventing heavy fogging. However, it's worth noting that the rear and side windows heat up more slowly, so it takes longer to completely remove the fog. Whether using automatic or manual air conditioning, turning it on helps with dehumidification. Avoid setting the temperature to the coldest position, as this can cause frost to form on the outer surface of the windshield over time. Applying Anti-Fog Spray or Other Preventive Measures: Spray a small amount of defogging agent onto the car windows and wipe it clean to remove dirt and stains. While polishing the glass, a thin transparent protective film forms on the surface, effectively preventing water vapor from condensing and forming fog. This is especially useful in cold winters. Alternatives to anti-fog spray include dish soap, soapy water, glycerin, alcohol, or saline solution. After applying, let it dry and then wipe off any excess residue with a chamois or soft dry cloth. This can keep the windows fog-free for several days. Opening Windows for Airflow: If the fog inside the car is not too heavy, opening the side windows can create airflow between the inside and outside of the car, reducing the temperature difference and gradually eliminating the fog. The principle of this method is the same as using cold air from the air conditioning. However, this method has obvious drawbacks and cannot be used during rain, snow, on highways, or when the fog is heavy. Rear Windshield Heating: Most cars are equipped with a rear windshield heating function. The button location may vary, but the function is the same. Turn on the windshield heating switch, and the fog on the rear windshield will disappear in about a minute.
Having driven for decades, turning on the cold air is absolutely my go-to when the windshield fogs up. It rapidly cools and dehumidifies, especially on humid rainy days when fog obstructs visibility. I immediately turn the AC knob to the windshield defrost position, using high-speed cold air, and the fog clears within five seconds. Warm air can also work, but it first raises the glass temperature, temporarily making the fog thicker before it dissipates, increasing the risk of delay. I remember once driving at night during a heavy rainstorm when I tried warm air, and my visibility worsened, nearly causing an accident—since then, I’ve only trusted cold air. Regular maintenance, like keeping windows clean and avoiding bringing wet shoes into the car, significantly reduces the chances of fogging. Replacing the cabin air filter regularly also ensures high dehumidification efficiency.
