Why is there water mist in the headlights?

[NaomiFitz]

When the outside air is humid, it is sucked into the headlights, and the hot air is expelled. When the lights are turned off, the temperature inside the headlights drops, causing the humid air to condense into water mist, which adheres to the headlights. This is the reason for the fog inside the headlights. Below is relevant information: Temperature difference causes heavy fogging: The headlight assembly's rear cover is equipped with a vent pipe, through which thermally expanded gas is expelled. However, sometimes some moisture enters the vent pipe, resulting in slight fogging. This fogging phenomenon usually occurs in winter, rainy seasons, or foggy weather. Car washing or wading can cause fogging: Besides temperature differences, fogging inside the headlights is more commonly caused by improper car washing or wading. When cleaning the engine, use a wet towel instead of a high-pressure water gun. Additionally, frequent wading can easily cause fogging in the headlights. If the water depth exceeds the wheel level, water can enter the headlights through the vent pipe. Water inside the headlights not only affects illumination but may also damage the headlights.