Why does the human body feel hot and cold related to the weather?

Weather related to human body's cold and hot feeling belongs to tactile problem. Many people are used to using temperature as the only criterion to infer the human body's feeling of hot and cold (perceived temperature). In fact, the temperature perceived by the human body is sometimes far from the actual temperature.

For example, people often say that "the hot weather is in the dog days", but according to the meteorological data, the highest temperature in most areas does not appear in the dog days, but appears around the dog days. People think the dog days are the hottest because the "hot" at this time is added with "wet" and sultry. People also have this experience. Under the rotating electric fan, they often feel that the wind from the electric fan is very cold, but when you put the thermometer in front of the electric fan and blow it, you will find that the temperature of the thermometer has not dropped. People feel cool because the wind has changed the air humidity.

I feel cool just after swimming in summer. If it is windy, I will even tremble with cold. All these show that the influence of atmospheric environment on human body is all-round. In addition to temperature, humidity and wind are the main factors that determine the human body's cold and hot feeling, and these three factors can not be ignored. The humidity of air is a physical quantity indicating the degree of dryness and wetness of air, which can be expressed in two ways. One is absolute humidity, which is expressed by the mass (or pressure) of water vapor contained in air per unit volume, indicating the amount of water vapor contained in air. At a certain temperature, there is a maximum amount of water vapor that can be contained in a certain volume of air. When this limit is reached, the water vapor in the air is saturated, and the gas at this time is called saturated gas; The other way is relative humidity, which is expressed by the percentage ratio of the absolute humidity of air at a certain temperature to the density (or pressure) of saturated water vapor at the same temperature, indicating the distance of water vapor in the air from saturation. The greater the relative humidity, the closer the water vapor in the air is to saturation, and the fewer "vacancies" in the air for water vapor molecules to fill, the slower the evaporation. The influence of humidity on people's sense of cold and heat is determined by the relative humidity of air.

When the weather is hot, in order to keep the body temperature at about 37℃, the human body must constantly radiate heat to the outside world. It mainly secretes sweat from sweat glands, absorbs heat through evaporation and takes it away. Molecules of sweat run out from the surface of sweat and become water vapor molecules. If people are in a relatively static air environment, these water vapor molecules will stay near human skin and form an "insulating layer". The water vapor in the "insulation layer" is getting closer and closer to saturation (the relative humidity of the air is getting bigger and bigger), and as a result, the evaporation rate of sweat is getting slower and slower, which makes people feel sultry. At this time, if the wind blows or the electric fan is turned on, the "insulation layer" will be blown away, the relative humidity of the air around the human body will be reduced, and the evaporation rate of sweat will be accelerated, making people feel cool. When the temperature is 24℃, the humidity in the air has relatively little influence on the cold and hot feeling of human body, and people feel neither cold nor hot. The air at 24℃ takes away a part of the human body's heat, and the heat generated in the human body makes up for that part of the heat taken away by the air, so that the human body maintains a relative thermal balance and feels good. When the temperature is lower than or higher than 24℃, people will have obvious cold and hot feeling, and relative humidity plays a great role in the cold and hot feeling of human body. For example, when the temperature is 25℃ and the relative humidity is 30%, the human body does not feel cold or hot; If the relative humidity increases to 95% at the same temperature, the human body will feel stuffy.

According to the measurement, when the temperature is 17.8℃ and the relative humidity is 100%, the human body's cold and hot feeling is the same as when the temperature is 28.6℃ and the relative humidity is 20%. In addition to air temperature and humidity, wind speed is also a very important factor in human body's sense of cold and heat. In winter, in windy weather or sitting in a Mercedes-Benz convertible, people seem to feel colder, because wind energy blows away the air "insulation" around the human body and takes away heat. In a certain numerical range, generally, the greater the wind speed, the faster and more the human body dissipates heat.