There is a significant difference in the performance of car lights in rainy days and foggy days. The main reasons include the following points:

1. Scattering of light:
 - Foggy Days: Water droplets in the fog will scatter the light of the car lights, causing the light to be diffused, causing the visible distance of the lights to be greatly reduced. The driver may feel that the lights are not bright enough and the vision is blurred.
 - Rainy Day: Rainwater exists in the form of water droplets in the air, but its light scattering effect is relatively small. Especially when the rain is heavy, the water droplets will form running water on the car windows, which may cause certain line of sight. block.

2. Type of lighting used:
 - In foggy weather, using fog lights (if the vehicle is equipped) can illuminate the road from a lower position, reduce glare caused by reflections, and help enhance visibility.
 - On rainy days, low beam lights are usually used to avoid high beam lights causing greater glare due to reflection.

3. Road Reflection:
 - Rainy Day: The ground is slippery, which can enhance the reflection of the road surface. When the light of the car lights comes into contact with the ground, it will be reflected back to the driver's eyes, increasing the visibility of the road surface.
 - Foggy Day: Due to the heavy fog in the air, light is scattered around, resulting in less reflection and reduced effective visibility.

4. Driver visual adaptation:
 - On rainy days, the driver's vision is often affected by water droplets, and he will automatically adjust his vision and attention. On the other hand, on foggy days, the driver may rely more on the brightness of the car lights and the recognition of distance.

5. Light brightness and color:
 - Humidity and water droplets in foggy days will affect the brightness perception of the light. In particular, white light is weaker in fog, making the light appear not bright enough; while red light and yellow light propagate better in fog.

In summary, the difference between rainy days and foggy days is mainly due to factors such as moisture distribution, light scattering, light use methods and environmental reflection, which make the performance of car lights under these two weather conditions completely different. This is why when driving in foggy weather, special attention should be paid to the use of lights to ensure driving safety.