Is the ultraviolet ray strong in sunny days or cloudy days? Why?

The ultraviolet rays in sunny days are stronger than those in cloudy days.

The ultraviolet intensity is divided into 5 levels: 1 level is the weakest, which usually indicates rain; Level 2 is weak and generally cloudy; Level 3 is medium, generally cloudy, and occasionally a little sun can be seen from the clouds; Level 4 is strong, generally sunny; Level 5 is the strongest, which usually means that the weather is particularly sunny.

The ultraviolet index refers to the degree of possible damage to human skin caused by ultraviolet radiation in the solar rays reaching the earth's surface when the sun is at the highest position in the sky (usually around noon, that is, from 10 to 3 pm). The variation range of the ultraviolet index is indicated by the number 0- 15. Usually, the ultraviolet index is 0 at night, and it is 15 in tropical, plateau and sunny days. The higher the UV index, the more serious the erythema damage to human skin caused by UV radiation. Similarly, the higher the ultraviolet index, the greater the damage to the skin in a short time.