Why is the temperature hot and the face yellow and black?

People's skin will appear yellow and black in hot weather, mainly because melanin is at work.

Melanin is a kind of protein. In the basal cells of human skin, there is a substance called pigment mother cells. When ultraviolet rays irradiate the skin, he can secrete melanin. Melanin can activate the activity of tyrosinase and react with tyrosine to produce a substance called dopa. In fact, dopa is the precursor of melanin, which will gradually appear on the skin surface after cell metabolism, causing dark spots and sunburn.

Black will appear in summer, mainly because of strong ultraviolet rays, which activate dopa in the human body and form melanin.