Why Does a Car's Front Windshield Fog Up on Rainy Days?

In daily life, it is a common phenomenon that the front windshield of a car tends to fog up on rainy days. This can be explained from the perspective of the principles of physics and the specific environment inside and outside the car.

The root cause of the windshield fogging is the condensation of water vapor. On rainy days, the temperature outside the car is generally lower. Meanwhile, the interior of the car is a relatively closed space. People in the car breathe out warm and moist air, and there may also be moisture from wet clothes or other sources. The warm and moist air inside the car comes into contact with the cold windshield. According to the principle of thermodynamics, when the warm and moist air meets the cold surface of the windshield, the air near the windshield cools down rapidly. As the temperature drops, the air's ability to hold water vapor decreases. Once the air is cooled to its dew - point temperature, the excess water vapor in the air will condense into tiny water droplets on the surface of the windshield, forming a layer of fog.

Another factor is the difference in air circulation inside and outside the car. On rainy days, drivers usually close the windows to keep out the rain, which reduces the air exchange between the inside and outside of the car. Without proper ventilation, the moisture - rich air inside the car has no way to be effectively discharged, and it continuously accumulates near the windshield. As a result, the probability and degree of fogging on the windshield increase. In addition, the cooling effect of the rain on the outer surface of the windshield makes the temperature difference between the inside and outside of the glass more significant, further promoting the condensation of water vapor and the formation of fog.

In conclusion, the combination of the temperature difference between the inside and outside of the car and the accumulation of moisture - rich air inside the car due to limited air circulation are the main reasons why the front windshield of a car is prone to fogging on rainy days.