Foggy windshield in the rain, and how to clear it
First find which side the fog is on. Wipe a fingertip across it from inside the car. If it clears, the fog is inside: aircon on, fresh air, fan at the glass. If it does not, the fog is outside: use the wipers and make the cabin a little less cold.
Both kinds clear faster on clean glass. Film on the inside and old water marks on the outside are what our windshield buffing service and a full interior clean take off, at your address.
Why it fogs here
Fog on glass is condensation. Warm, wet air touches a surface colder than itself and leaves water on it as tiny droplets.
Most of the advice online is written for cold countries, where the warm wet air is inside the car and the glass is cold from outside. In a Philippine downpour it can run the other way. The air outside is warm and close to saturated, the aircon has chilled the glass, and the droplets form on the outside of the windshield. That is why the defroster seems to do nothing.
The inside fogs too, from wet passengers, wet umbrellas and the aircon turned off to warm up. Usually you get one side at a time, and the fix for one is close to the cause of the other.
Fog on the inside
- Turn the aircon on with the fan up and point it at the windshield. Aircon dries the air as it cools it, which is what clears fog from the inside.
- Switch from recirculate to fresh air. Recirculated air keeps the moisture from everybody breathing in the car.
- If it is still slow, open a window a little until the glass clears, then close it.
- Wet umbrellas and wet clothes on the floor keep putting water back into the air. Put them in the trunk if you can.
Fog on the outside
- Use the wipers, with a spray of washer fluid if it smears. Fog on the outside wipes off; the defroster will not touch it.
- Raise the temperature setting a few degrees or turn the fan down. The glass is fogging because the aircon has made it colder than the air outside.
- Do not switch the aircon off completely in heavy rain. The inside will fog instead, and that is the side the wipers cannot reach.
Why dirty glass fogs faster
A dirty window fogs more readily than a clean one, because the dirt gives the water something to form on. The haze on the inside of most windshields is dust, smoke and whatever the dashboard gives off in the heat. You can smear it with a fingertip. It comes off with a glass cleaner and a clean microfiber cloth, and it wipes off during an interior clean.
The outside matters in a different way. Water marks and a worn band across the wiper sweep scatter the light from oncoming headlights, so a windshield that is only a little fogged becomes very hard to see through at night. Those marks need a glass polish. Which ones come off is covered on acid rain marks on a car and windshield scratches.
Anti-fog sprays for the inside of the glass are sold for this and work on glass that is clean and dry when they go on. Applied over the film, they mostly spread it around.
Book windshield buffing at your address
If the glass glares at night even when it is not fogged, read what our windshield buffing service covers. For the inside of the glass along with the rest of the cabin, that is the interior and exterior detail. Tell us in the form which one is bothering you.
Sources
- Smiley's Glass, how to defog a windshield in summer humidity: warm air on a cold surface, the aircon on a humid day, wipers and washer fluid for fog on the outside, a dirty window fogs more readily, anti-fog treatments on clean dry glass. smileysglass.com