What Causes Fogging in Headlights?

[IslaAnn]

The reason for fogging in headlights is: when the car headlights are turned on, they generate a significant amount of heat, causing the air inside the lamp housing to warm up. When the engine is turned off and the car is parked, especially at night, the headlight interior cools down from hot to cold, causing water vapor in the air to condense on the lamp housing, forming fog. Car headlights are designed with a semi-sealed structure. The headlight housing is sealed, but there is a vent hole on the headlight assembly to prevent dust. When the weather is hot, or under sunlight, or after the headlights have been on for a while, the fog will evaporate and exit through the vent hole. There is also an abnormal situation where headlights may have water ingress, which occurs when a large amount of water enters. Some headlights have poor sealing and are not completely airtight. When the headlights are in operation, they generate a lot of heat, causing the internal temperature to rise and the air pressure inside the housing to increase. This can force open any existing gaps in the seal, allowing rainwater to flow into the housing through these gaps.