What does it mean when it’s cold at 39 and hot at 3 volts?

Because the earth's axis of rotation is tilted at a certain angle, there are seasonal changes of hot and cold as it orbits the sun. However, in most parts of China, the coldest and hottest periods of the year are not the winter solstice and the summer solstice, when the sun has the lowest altitude (the weakest solar radiation) and the highest (the strongest solar radiation) throughout the year. Instead, they tend to be later. It is "cold in the 39's and hot in the 3 volts".

The time of "March Nine" and "Sanfu" is:

"March Nine": Counting from the winter solstice to the third nine days, roughly between 9 and 9 of January 17th.

"Three Volts": The time varies every year. Generally, the first period is in mid-to-late July, the middle period is in late July and early August, and the last period is around mid-August.

Because the rise and fall of temperature depends not only on the heat income, but also on the expenditure of heat radiated from the ground to space. When the two reach a balance, the temperature can turn around and rise or fall. Although the solar radiation received by the ground begins to decrease after the summer solstice, it is still greater than the expenditure, and the temperature continues to rise until the balance reaches balance ("three volts"). Similarly, between the winter solstice and the "March 9th", the temperature continues to drop because the ground heat expenditure is still greater than the income, and it does not start to rise until the "March 9th". However, this experience also has regional limitations. When it comes to the Nansha Islands, which is close to the equator, the "dog period" is one of the two coolest seasons of the year, although the temperature changes there throughout the year are very small. Therefore, "the heat is in the third volt" is not "the heat is in the third volt (the last volt)", nor is it "the heat is in all three volts", but mainly "the heat is in the middle volt". It is said that "hot is in the third volt" just to contrast with "cold is in the third and ninth", so that it is easy to remember. The reason why coastal areas and islands are "heated at the end of the day" is because the heat capacity of seawater is greater than that of land soil, and the temperature rises slowly in spring and summer, so it is later to reach heat balance and reach the highest temperature. Taking Shandong as an example, inland Dezhou is the hottest in the year in late July, while in Chengshantou on the tip of the Shandong Peninsula at the same latitude, it is as late as mid-August. By the same token, the coldest period of the year on coastal islands should also be postponed. But the strange thing is that Chengshantou is still basically "cold in Sanjiu". It turns out that this is because the cold air coming south from Siberia in winter is too strong, and it is difficult for the northern coast to show this kind of ocean adjustment. It doesn't start to appear until south of the Yangtze River, such as east of Shanghai and the Shengsi Islands in the East China Sea. The coldest is in early February. But even in areas where "cold is in the 39s and hot is in the middle", there are a few years where this is not the case. From August 18 to 24, 1959, the middle and lower reaches of the Yangtze River experienced seven consecutive days of extremely high temperatures that were rare in history (and certainly the hottest in the whole year of 1959). For example, the daily average temperature in Wuhan was as high as 32.4 to 33.4°C, causing many people to suffer from heat stroke. This is the famous "Autumn Tiger". On the contrary, the Meiyu period in 1978 was extremely short and the drought period came earlier. For example, the average temperature in Hangzhou in early July was 32.9°C, and the highest temperature in the afternoon was around 39°C. It was a rare high temperature weather in history. This is what the meteorological department calls "empty plum fever."