Using Cold Air or Hot Air for Defogging?
3 Answers
Defogging can be achieved using both hot air and cold air. There are three methods for defogging. Cold Air Defogging: The fundamental reason for window fogging is the significant temperature difference between the inside and outside of the car—cold outside and warm inside, which causes condensation on the windows. Using cold air for defogging can quickly dry the moisture on the windshield. Therefore, for emergency defogging while driving, cold air is effective. Hot Air Defogging: When using hot air for defogging, the fog on the windows may initially become heavier, but after a while, the hot air will dry it out. Since the moisture is removed, the windows will remain fog-free for a long time, and the interior of the car will also become much warmer. Opening Windows or Using Defogging Spray: Another method is to open the car's sunroof (or windows) while driving to maintain a consistent temperature inside the car, preventing fogging. Alternatively, you can mix dish soap with water, spray it evenly on the glass, and then wipe it clean with a dry cloth. This provides long-lasting defogging without leaving any marks.
As an automotive technology enthusiast, I've been pondering the principles of defogging. Fogging mainly occurs due to high air humidity, and using the air conditioning (cooling) is more effective because it rapidly dehumidifies through the AC evaporator, condensing moisture inside the cabin and instantly dispersing the fog. While hot air can also evaporate moisture, its efficiency is lower in humid conditions and may require the engine to warm up before becoming effective. I recommend directly pressing the AC defog button to activate the AC mode, as automatic cooling and dehumidification are more reliable. This is particularly useful on rainy or muggy days, and I've also noticed that turning on the recirculation mode can accelerate the dehumidification process. In short, cooling is the preferred option—simple, safe, and without added risks.
Having driven for over a decade, I handle defogging based on experience. When fog appears on rainy days, I immediately turn on the AC cold air and direct it at the windshield—it clears up in seconds, much faster than using hot air. Hot air requires waiting for the engine to warm up, which is fine in winter but a waste of time in summer. I did the same in my old car, but modern cars make it even simpler—just press the defog button to activate the cooling system. I remember once during a business trip when heavy fog made the road invisible; the cold air cleared it instantly, whereas hot air only made things worse. For regular maintenance, I keep the AC clean to ensure the defogging function works properly—this trick has saved me in many urgent situations.
