Physics problems, physics experts came. Xiaoming traveled to Kunlun with his parents in the summer vacation and got off the plane. His sister, the tour guide, said to them, "Kunlun. ...

Analysis: Because Kunming is located in the plateau, even on cloudy days, there are a lot of invisible ultraviolet rays outside, which are harmful to human body, making people's skin black, shallow and rough, and even causing skin cancer.

Answer: it is because there are a lot of invisible ultraviolet rays outside that irradiate the human body, making the skin black and rough.