When the weather is hot, people's facial skin will turn red, because blood vessels are small.

When the weather is hot, people's facial skin is red, which is because blood vessels are small (dilated) and help to dissipate heat; When the weather is cold, people will be "pale", which is because blood vessels are small (contracted), which helps (preserve) heat; When the temperature is extremely cold, people's facial skin will freeze red, which is because the blood vessels are small (under the self-protection effect, blood circulation is accelerated), which helps to (generate) heat, thus preventing skin (frostbite).