Because the earth's rotation axis tilts at a certain angle, the four seasons change when it revolves around the sun. However, in most parts of China, the coldest and hottest time of the year is not the winter solstice and summer solstice with the lowest solar height (the weakest solar radiation) and the highest solar radiation (the strongest solar radiation), but often postponed, that is, the proverbial "cold in 39 and hot in 3 volts".
"Three nine" refers to counting from the winter solstice to the third nine days, roughly from 9 to 17 January. The "three volt" time varies from year to year. Generally, the first volt is in the middle and late July, the middle volt is in the late July and early August, and the last volt is around the middle of August.
Why is the coldest and hottest period of the year postponed to "March 9th" and "Sanfu"?
This is because the rise and fall of temperature depends not only on the income of heat, but also on the expenditure of heat radiated from the ground to space. When the two reach equilibrium, the temperature can turn around and rise or fall. Although the solar radiation on the ground began to decrease after the summer solstice, it was still greater than the expenditure, and the temperature remained rising until the revenue and expenditure reached balance ("three volts"). Similarly, between the winter solstice and March 9th, the temperature also continued to decline because the ground heat expenditure was still greater than the income, and did not begin to rise until after March 9th.
However, this experience also has regional limitations. The Chinese mainland is mainly hot in mid volt and head, but coastal areas (within 30~50 km of the coast) and islands are hot at the end. The hottest time in Yunnan even occurred in May before the rainy season, which was two months before the fall. In the equatorial Spratly Islands, the "three volts" period is one of the two most cool seasons in the whole year, though the annual temperature change is very small. In the "third volt", not in the "third volt". Saying "hot in three volts" is just to compete with "cold in three nine". It's easy to remember.
The reason why coastal and island "heat is at the end" is that the heat capacity of seawater is larger than that of land soil, and the heating rate is slow in spring and summer, so it is late to reach the balance of heat budget and the highest temperature. Taking Shandong as an example, the hottest day of the year in Dezhou inland is in late July, while Chengshantou on the tip of Shandong Peninsula at the same latitude is late to mid August.
Similarly, the coldest time of the year on coastal islands should be postponed. Strangely enough, Chengshantou is still basically "cold in March and September". It turns out that this is because the cold air from Siberia to the south in winter is so strong that it is difficult to show this kind of marine regulation along the northern coast. It didn't appear until the south of the Yangtze River. For example, Shengsi islands in the East China Sea, east of Shanghai, were the coldest in early February.
But even in areas where "the cold is in March and September and the heat is in the middle", there are a few years that are not the case. From August 18 to 24, 1959, the middle and lower reaches of the Yangtze River experienced high-temperature weather for seven consecutive days (which was, of course, the hottest in 1959). For example, the daily average temperature in Wuhan was as high as 32.4 ~ 33.4 ℃, resulting in heatstroke for many people. This is the famous "autumn tiger". On the contrary, the plum rain period in 1978 was very short and the summer drought period was advanced. For example, the average temperature in Hangzhou in the first ten days of July was 32.9 ℃, and the highest temperature in the afternoon was about 39 ℃, which was a rare high-temperature weather in history. This is what the meteorological department calls "empty plum fever".