1. Sun exposure: When drying the back, the sun shines directly on the back, which makes the back stimulated by high temperature, but the neck is not exposed to direct sunlight, so it will feel relatively cool.

2, sweating on the back: when the back is dry, the back is easy to sweat, and the evaporation of sweat will take away some heat, which will make the back feel cool, while the neck will feel relatively cold because there is no sweat.

3. Blood circulation: When the back is dry, the blood vessels in the back will expand due to the sunlight, which will accelerate the blood circulation, while the blood circulation in the neck is relatively slow due to the lack of direct sunlight, which will cause the neck to feel cold.