Today is the beginning of the dog days丨Why are the dog days so hot?

Today marks the beginning of the dog days of summer, which is the hottest, most humid and muggy time of the year.

Where does the term "Three Dog Days" come from? What makes this period so hot? How can we "survive" with a smile during the Three Dog Days? Let's find out together!

Which are the three hot days?

The term "Three Fu" is extended from the Five Elements Theory which was popular during the Qin and Han Dynasties. It mainly refers to the hottest period of the year in the Central Plains. It is a general term for "the first Fu", "the middle Fu" and "the last Fu", which occur from mid-July to mid-August in the Gregorian calendar. At this time, people will sigh, "All I need to do to get barbecue is a pinch of cumin."

Image source: Internet

"Fu" means that the yin energy is forced to hide underground by the yang energy. The third Geng day after the summer solstice is the beginning of the first Fu (first Fu), the fourth Geng day is the beginning of the second Fu (second Fu), and the first Geng day after the beginning of autumn is the beginning of the last Fu (third Fu).

So, what does "Geng Day" mean? A "Day Recording Method" has been popular in my country since ancient times. That is, one character is taken from each of the Heavenly Stems "Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren, and Gui" and the Earthly Branches "Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu, and Hai" to record the days. The days with the character "Geng" are called "Geng Day". Since there are 10 Heavenly Stems, there is a Geng Day every 10 days. The third Geng Day after the summer solstice is the beginning of the dog days. In this way, the dog days have fixed dates.

This year's first dog days starts on July 11, the middle dog days last from July 21 to August 9, a total of 20 days; the last dog days last from August 10 to August 19. The 40-day dog ​​days period really makes this summer seem particularly long!

Causes of the dog days

The Sun: The Biggest "Mastermind"

Speaking of the dog days, the scorching sun must come to mind first. However, the sun cannot be blamed casually. The dog days are indeed closely related to it.

As we all know, our earth is constantly rotating and revolving around the sun, and the earth is rotating sideways. It is thanks to this unique configuration that there are day and night and four seasons on the earth. The sun is the main source of energy for the earth's atmosphere. China's 24 solar terms are determined by the different positions of the earth's revolution.

GIF: The sun shines directly on the earth in one year (from the Internet)

In spring and autumn, the sun shines directly on the equator. During the summer solstice, the sun shines directly on the Tropic of Cancer. At this time, the Northern Hemisphere receives the most solar radiation energy, and it is summer in the Northern Hemisphere. During the winter solstice, the sun shines directly on the Tropic of Capricorn. At this time, the Northern Hemisphere receives the least solar radiation energy, and it is winter in the Northern Hemisphere. In essence, it is the sun that dominates the Earth's climate. So between summer and autumn, we in the Northern Hemisphere are naturally in the "palm" of the sun.

Subtropical High Pressure: The Main Accomplice

In addition to the sun, there is another "murderer" that makes the dog days of summer particularly hot: the subtropical high pressure.

It appears in the subtropical regions of the northern and southern hemispheres. In the northern hemisphere, the subtropical high pressure slowly moves northward as the sun's direct point moves northward.

Schematic diagram of the subtropical high pressure belt (from the Internet)

The area covered by it will be controlled by the downdraft, and the sky will no longer be able to accumulate many clouds, making it exceptionally clear, causing the solar radiation received by the surface to rise sharply, and the temperature will also rise accordingly. However, in the northwest edge of the subtropical high pressure, some southerly air currents blow warm and humid air from the ocean to the Chinese mainland, providing water vapor for rainfall. As the surface humidity increases, the heat dissipated will decrease. In this way, with increased heat absorption and reduced heat dissipation, the surface heat continues to accumulate, and the situation becomes hotter day by day.

When the dog days of summer come, the accumulated heat on the ground reaches its peak and the weather is the hottest . This is why, although the summer solstice has the highest solar altitude angle, it is not the hottest period of the year - the accumulated heat has not yet reached its peak.

Global warming: To blame!

Hot weather always makes people irritable, and some people begin to point the finger at the greenhouse effect caused by human production and life. So, did ancient people not experience such high temperature weather? Historical climate research shows that the hottest summer in China's history occurred in the eighth year of Qianlong. The "Compendium of China's Three Thousand Years of Meteorological Records" says that at that time, Beijing, Tianjin, Hebei, Shanxi, Shandong and other North China regions were abnormally hot; the actual temperature recorded by French missionaries in Beijing that year (1743) was that the temperature values ​​from July 20 to 25 were all above 40℃, and on July 25 it reached an astonishing 44.4℃! There were no electric fans and air conditioners at that time, and the ancients suffered more than us! Of course, it is difficult to say that such extreme weather events are directly related to global warming.

Ancient people would dig ice in winter and store it to cool down in summer (Source: Internet)

However, asphalt roads, cement roads and various reinforced concrete buildings in modern cities also contribute to the increase in ground temperature by changing the reflectivity of the ground surface. This is also the reason why the temperature in the central city is higher than that in the suburbs, resulting in urban heat islands. It can be seen that the high temperatures we are "enjoying" now are the joint product of natural climate and "artificial processing".

Which of the three hot seasons is the hottest?

In a specific year or region, whether the highest temperature occurs in the first, second or last futu, or whether the "autumn tiger" will appear after the last futu of the beginning of autumn, actually depends on the weather system at the time. Usually in mainland my country, the heat is mainly in the first and middle futu, but the southeastern coastal areas and islands are mostly hot in the last futu; the hottest period in Yunnan is May before the rainy season, two months before the beginning of futu; in the Nansha Islands of my country, the third futu is one of the two coolest seasons of the year. Even in the middle and lower reaches of the Yangtze River, which is usually hot in the middle futu, continuous high temperature weather known as the "autumn tiger" may occasionally occur in mid-to-late August after the last futu.

Stay healthy during the "Three Dog Days"

Although the dog days of summer are really hot, it is actually a normal climate phenomenon. Every year we should welcome and deal with it with a calm attitude, seek benefits and eliminate harm, and minimize its adverse effects as much as possible:

Reduce outdoor work on hot days and strengthen heatstroke prevention and labor protection measures;

• Treat winter diseases in summer and go to regular medical institutions;

• To prevent heat stroke, do not seek coolness; eat more bitter melon and less cold drinks;

There is a saying among the people that goes "Eat dumplings in the first dog days, eat noodles in the second dog days, and eat pancakes and eggs in the third dog days." In short, as long as we respond positively and correctly, we can all get through the three dog days in good health and safety.