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Short Summary
Main recommendation: To defog your windshield fastest, turn the heater on full blast, turn the AC on, turn off inside air circulation, and crack your windows slightly if conditions allow. Mark Rober says this combination defogged windows in less than half the time of other settings in his 10-morning test. The logic is that hot, dry air can absorb more moisture, and the AC pulls moisture out of the air.
Key Points
- The four-step list promised by the title, in order: (1) heater on full blast, (2) AC on, (3) inside air circulation off, (4) crack windows slightly.
- Fog is condensation of water vapor. Warm air holds far more moisture than cold air; the video cites about 53 ml at 103°F versus about 5 ml at 36°F at sea level.
- Rober models air as a towel: its size is temperature, and its initial wetness is humidity. A large, dry towel absorbs fog fastest.
- Recirculating inside air is a problem because cold winter air that is brought in and heated still holds the moisture from inside the car.
- Anti-fog liquid helped, but performed no better than Jurassic Park shaving cream, so it is not worth the extra cost in his test.
Details Worth Noting
- Concrete instructions:
- Turn the heater to full blast.
- Turn the AC on; it pulls moisture out as air passes over the cold coil.
- Make sure inside air circulation is off, so outside air is used rather than recirculated air.
- Crack as many windows as possible, even for a few seconds, to exchange humid cabin air for dry outside air. This may not be possible in freezing or rainy weather.
- If fog persists, place kitty litter in a sock near the windshield to absorb moisture.
- Results will vary with current weather and car type, so compare these settings against your usual method.
- Rober tested for 10 mornings with the help of trusted friends, documenting and ranking combinations of settings.
- He says the setup is a simplification of air's moisture-holding behavior.
- The anti-fog liquid and shaving cream comparison was done on separate portions of the same windshield in one test.
Bottom Line
Heater on full, AC on, recirculation off, and windows cracked when safe is the combination Rober found cleared fog in under half the time, and it is cheap to try.
