Sydney feels very hot at 23 degrees in summer. Why?

The hot summer has arrived, and many areas have ushered in the weather of alternating high temperature and rainfall. Whether in the northeast or the south, there is not much difference in the temperature in midsummer, but friends who travel frequently have probably felt this way. For example, when the temperature is almost the same, when we go to the coast and inland, there will be a very strong difference in somatosensory temperature. In coastal areas, we will feel breathless and sultry. Even if the temperature is the same as that in inland areas, our coastal areas are much less tolerant of high temperature. Then, why do coastal areas with high air humidity give us a warmer feeling when the temperature is the same in summer?

Human body heat dissipation system

Humans, like other advanced mammals, are warm-blooded animals, which is an efficient physiological regulation mechanism formed in the long-term natural evolution process. Because the human body is engaged in physiological activities all the time, it needs to consume corresponding energy, and a large part of it is released in the form of metabolism and heat.

In human skin, there are many receptors that can feel external environmental factors and changes, including baroreceptors that can feel touch, pain receptors that can feel strong stimulation, and temperature receptors that can feel hot and cold. For the external environment with a temperature higher than the body, thermal receptors can be triggered.

When these temperature receptors are stimulated by external temperature conditions, they will send cold or hot signals to the brain through the thermoregulation center, thus affecting and changing the activity state of endocrine, bones, blood vessels, sweat glands and other tissues and organs, thus adjusting the process and efficiency of body heat release and heat dissipation, and finally keeping the body temperature at a relatively stable level, that is, about 36.3-37 degrees Celsius.

The evaporation of sweat on the surface of human skin can be divided into several situations. One is non-inductive evaporation, that is, you can hardly feel the evaporation of water when sweating, and there is almost 600-800 ml evaporation every day. When the external environment temperature reaches 25-30 degrees Celsius, it will generally stimulate sweat glands. At this time, sweating is not the main way of heat dissipation, and thermal radiation and heating the air around the body to form thermal convection still account for more than 60% of heat dissipation. However, when the ambient temperature is equal to or higher than the body temperature, thermal radiation and thermal convection are ineffective. At this time, the human body can only dissipate heat by sweating.

There are many factors that affect the heat dissipation rate of human body, the most important of which is the difference between human body temperature and external environment temperature. The greater the difference between skin temperature and ambient temperature, the faster the heat dissipation rate of human body. In addition, the content of water vapor in the air and the air velocity also have great influence on the heat dissipation efficiency.

wet-bulb temperature

I believe some people have never heard of this term. The temperature we usually see in weather forecast or measured by thermometer is actually the real temperature of air, also called "dry bulb temperature", which is a scalar to measure the real temperature of an object or environment.

However, in some cases, "dry-bulb temperature" can't fully experience people's actual feelings about temperature. For example, in the coastal areas with high air humidity in summer, the somatosensory temperature at the same temperature is much hotter than that in the mainland. For another example, after a thunderstorm in summer, the weather becomes clear again, and we will feel hotter. This is a truth. At this time, it is necessary to create a scalar that can measure the somatosensory temperature, which is different from the "dry bulb temperature" and the "wet bulb temperature" came into being.

In meteorology, "wet bulb temperature" refers to the air temperature when the water vapor content in the air reaches saturation. On the other hand, the "wet bulb temperature" can also represent the lowest temperature that we can reach only through the evaporation of water in the current environment.

When measuring the ambient temperature with "wet bulb temperature", we can not only know the ambient temperature, but also consider the factors such as temperature, wind speed and solar radiation intensity, because these factors all play a vital role in the evaporation of water. Through the wet bulb temperature, we can more accurately describe the human body's feelings about the weather and the surrounding environment. Therefore, meteorologists now use "wet bulb temperature" more and more to express the influence of temperature change on human body.

The following are some experimental values, which you can have a look at, describing the tolerance time of human body directly exposed to the environment at the corresponding "wet bulb temperature". For example, when the "wet bulb temperature" is between 26-3 1 celsius, the body pressure will increase exponentially within 30 minutes; When it exceeds the "wet bulb temperature" of 3 1 celsius, outdoor sports can last for up to 20 minutes; When the "wet bulb temperature" of 32 degrees Celsius is exceeded, this time will be shortened to 15 minutes.

Explain the sultry weather with "wet bulb temperature"

When the humidity in the air is low, the sweat discharged from the body can evaporate at a very fast speed. Even if the ambient temperature exceeds the body temperature, you can still cool down smoothly through sweat. Therefore, in "dry and hot" weather, the body can generally adjust itself.

But when the humidity in the air gradually increases, so that the "wet bulb temperature" of the environment exceeds 35 degrees Celsius, the situation will change obviously. At this time, the evaporation rate of sweat on the skin surface has become very small, and the heat generated in the metabolic process, whether through thermal radiation, thermal convection or sweat evaporation, is almost "blocked", and the heat cannot be emitted from the inside of the body, thus increasing the burden on the heart, lungs and blood vessels. At this time, I feel stuffy.

Therefore, the environmental "wet bulb temperature" is bounded by 35 degrees Celsius. Once it is higher than this value, it will bring a certain degree of threat and danger to the human body. In recent years, the phenomenon of supercritical high "wet bulb temperature" has become more and more frequent in the world and its influence has become wider and wider. For example, the 20 10 "heat wave" in Russia and the "heat wave" in Europe in 2003, when the "wet bulb temperature" was lower than 29 degrees Celsius, even so, 55,000 people and 35,000 people died respectively.

Previously, climatologists believed that the "wet bulb temperature" on the earth could not exceed 3 1.7 degrees Celsius. However, last year, some research institutions showed through a large number of monitoring data that this originally thought limit point had been broken through many times. Only at the two monitoring points in this study, there have been many times when the "wet bulb temperature" exceeded 35 degrees Celsius.

Scientists have judged that the areas experiencing extreme "wet bulb temperature" on the earth are mainly concentrated in South Asia, the Middle East coast, the western United States and other areas near the ocean. The air humidity in these places is high in summer, which is easy to overlap with high temperature weather. This problem will also occur in the southeast coast of China from July to August every year. Although the degree is not as strong as the above areas, this kind of weather is really hard. With the global warming, extreme weather events become more and more frequent. In case of high wet bulb temperature, outdoor activities should be reduced as much as possible, and heatstroke prevention and cooling should be done effectively in time.