Dark skin and ultraviolet ray

Among the places I visited when I was in Yunnan was Li Jiang - home of the Naxis, one of the minorities there. Most of them look dark. The tourist guide then explained why they looked dark.
As they live in a place 2400 meters above sea-level, the sun's rays take a shorter distance to reach their bodies, thereby darkening their skins through the ultraviolet rays.

The Naxian girl would like to be fat as it is considered beautiful. In fact, all Naxian girls call themselves 'Pang Jin Mei' or Fatty Sister and all men are called 'Pang Jin Ge' or Fatty Brother.

I felt quite hot even though the temperature was 6 degrees when the sun was out. It must the heat that does the work of darkening the Naxians.