Should You Use Hot Air or Cold Air for Defogging Car Windows?

[McElliana]

Both cold air and warm air can be used to defog car windows, but warm air is slower than cold air. The most effective method is using cold air, which blows dry, cool air directly onto the glass, preventing water vapor from condensing and achieving defogging. Warm air works similarly but should be avoided in rainy conditions as it may initially worsen fogging. When defogging, always use the car's external air circulation. The cause of car window fogging: A temperature difference exists between the inside and outside of the car. If the temperatures differ, the side with the lower temperature has a lower saturated vapor pressure, causing moisture to condense on the glass surface, forming tiny droplets that create fog. Methods for defogging car windows: Air Conditioning Cooling: Use the air conditioner's dehumidification function to reduce air humidity and remove fog. In summer, especially when multiple passengers enter the car without turning on the AC, exhaled humid air can quickly fog the windshield. Turning on the AC to blow cold air onto the windshield can dehumidify and clear fog quickly. If humidity is too high, slightly opening a window can speed up the process by reducing cabin humidity. Combining both methods yields faster results. Air Conditioning Heating: Reduce fogging by minimizing temperature differences. In winter, blowing warm air onto the windshield raises its temperature, reducing the temperature difference between the inside and outside surfaces and preventing heavy fogging. Note that rear and side windows heat slower, requiring more time to clear completely. Whether using automatic or manual AC, turning it on aids dehumidification. Avoid setting the AC to the coldest temperature to prevent external windshield fogging over time. Applying Anti-Fog Products: Spray a small amount of defogger onto the car windows and wipe clean to remove dirt and stains. This leaves a transparent protective film that prevents moisture condensation, ideal for cold winters. Alternatives like dish soap, soapy water, glycerin, alcohol, or saline can be applied, left to dry, and then wiped with a soft cloth to keep windows fog-free for days. Opening Windows for Airflow: If fogging is mild, opening the side windows allows air convection, reducing the temperature difference and gradually clearing the fog. This method works similarly to using AC cold air but has clear drawbacks: it’s unsuitable during rain, snow, high-speed driving, or heavy fog. Rear Window Heating: Most cars have a rear window heating function, with button locations varying by model. Turning on the defroster clears rear window fog in about a minute.