【Creative Cultivation Program】Why do you feel hot when the temperature is 30 degrees, but feel cold when taking a shower with 30-degree water?

Author: Wang Pingguo

Reviewer: Ma Zansong, Deputy Director of the Department of Traditional Chinese Medicine, Shanghai Fifth People's Hospital Affiliated to Fudan University

The weather is getting hotter and hotter. Looking at the temperature outside which is over 30 degrees, it is really hard to have the desire to go out. It is rare to go out, and you are sweating again. So you decide to take a shower and adjust the water temperature to about 30 degrees. After a refreshing shower, you suddenly realize a problem: why does 30-degree bath water make people feel cool, while 30-degree air temperature makes people feel hot?

To understand this problem, let's first look at how the human body perceives temperature.

We all know that humans are homeothermic animals. Homeothermic animals are animals whose body temperature does not change due to the temperature of the external environment and always remains relatively stable. The human body temperature is generally constant between 36 and 37 degrees.

What maintains this temperature is our "hypothalamus". Current medical research has confirmed that the body temperature regulation center is located in the anterior medial preoptic area of ​​the hypothalamus, which cooperates with our body's temperature receptors to complete the regulation of body temperature.

There are many temperature receptors on the surface of our body, mainly located in the skin, mucous membranes, etc. These receptors can be divided into two types: cold receptors and warm receptors. They transmit the temperature changes of the skin and the external environment to the body temperature regulation center. In real life, when the skin temperature is 30 degrees, a cold sensation will occur, and when the skin temperature is 35 degrees, a warm sensation will occur.

Since a skin temperature of 30 degrees will produce a cold feeling, why does 30-degree weather make people feel hot?

In fact, air temperature and the temperature we feel on our body surface are not the same concept. In summer, the perceived temperature is usually higher than the air temperature.

The temperature we usually see in the weather forecast refers to the temperature in the louvered box at a height of 1.5 meters in the weather station, but our daily living environment is not the same as that of the weather station. In addition, the temperature we feel most directly on our body surface is what affects us the most. In addition to being different for different people, the perceived temperature is also affected by conditions such as humidity, wind speed, and sunlight.

The most obvious thing is that if we stand directly under the sun in summer, we will feel very hot, but if we stand under the shade of a tree, we will feel much better. The temperature is actually the same at this time, but the temperature felt on the body surface is different.

Humidity also has a great impact on the perceived temperature. At the same temperature, the higher the humidity, the higher the perceived temperature. In summer, the humidity is usually very high, so the temperature we actually feel is much higher than the air temperature.

After answering why you feel hot at 30 degrees, let’s look at why 30 degrees bath water makes you feel cold.

This is mainly due to the different heat transfer capabilities of water and air.

For an object in a certain medium, the heat transfer coefficient in water is 230W/㎡·℃ (heat transfer coefficient unit, watt/(square meter·degree)), and the heat transfer coefficient in air is 9W/㎡·℃. The larger the value, the better the heat transfer performance. As you can see, the heat transfer capacity of water is much better than that of air, about 25 times that of air (230/9).

This means that when we are in contact with water, more heat will be absorbed by the water flow. In the air, due to poor heat transfer performance, the body heat is not easy to dissipate, so although the temperature is the same at 30 degrees, the difference between the two is still quite large. In addition, there is another point, when taking a bath, people usually do not wear clothes... Without the obstruction of clothes, the body's heat dissipation efficiency is higher and it will be cooler.

Finally, I would like to add a very interesting piece of knowledge. Although it is refreshing to take a cold shower in summer, the high heat transfer of water actually has disadvantages. I was scalded by hot water when I was a child. When the water temperature is much higher than the body temperature, the high heat transfer of water will also mercilessly bring more heat to the human body, causing burns, but there will be no big problem in the air. According to the records of the Sauna World Championship, the human body can last for 17 minutes in a 110-degree sauna room, but if it is put into 100-degree hot water, well, that scene is a bit unimaginable...

References:

Yang Zhi. Discovery of the central neural circuit for thermoregulation[J]. Science, 2017, 69(2):1.

Liu Jingyu. Effects of air humidity and wind on perceived temperature[J]. Meteorological and Environmental Sciences, 2003, 000(003):34-34.

This article is produced by the Science Popularization China-Creation Cultivation Program. Please indicate the source when reprinting