Why does the skin turn black obviously after returning to the countryside, even if it rains every day? Is there any scientific statement?

The reason why the skin turns black is that the intensity of ultraviolet rays is higher than that of cities.

Urban air pollution is greater than that in rural areas, and pollutants in the air reduce the radiation intensity of ultraviolet rays to human body.

After returning to the countryside, even if it rains every day, the intensity of ultraviolet rays is much higher than that of cities. Ultraviolet rays darken the skin, and proper irradiation of ultraviolet rays can accelerate the absorption of calcium, which is one of the reasons why urban residents like to have an outing.