What causes fogging inside car headlights on rainy days?

[ZoeMarie]

Headlight fogging is normal. This occurs because there is a ventilation and heat dissipation rubber tube at the rear cover of the headlight assembly. Moisture from the air enters the headlight through this rubber tube and condenses on the lens, forming fog. In severe cases, water droplets may appear. This phenomenon is most common during winter and rainy seasons. Below is relevant information: Mild fogging: First is mild fogging, which is solely related to air humidity. Typically, the amount of condensed moisture from headlight fogging is minimal. The headlight assembly has several intake and exhaust vents designed to balance the internal air temperature when the headlights are on, expelling hot air and drawing in cool air. Severe fogging: This is usually caused by cracks in the headlight assembly, improperly fitted dust covers, holes in dust covers, or aging sealant. After driving, when the headlight interior remains very hot, large amounts of highly humid air enter. If the outside temperature is relatively low, a layer of condensation will form inside the headlight assembly.