Why is the weather in Hunan hot and cold?

Climate characteristics

Hunan has a continental subtropical monsoon humid climate. This is closely related to the fact that this province is located between 24 39' and 3 8' north latitude, living in the southeast of Eurasia, facing the Pacific Ocean, and the border of Southeast Asia is 4 kilometers away from the sea, which is closely related to the influence of East Asian monsoon circulation. Hunan's climate has the following three characteristics: first, light, heat and water resources are abundant, and their high values are basically synchronous. From April to October in the whole province, the total radiation accounts for 7-76% of the total annual radiation, while the precipitation accounts for 68-84% of the total annual precipitation. Second, the climate changes greatly within and between years. Winter is cold and summer is hot, spring temperature is changeable, autumn temperature drops sharply, spring and summer are rainy, and autumn and winter are dry. The interannual variation of climate is also great. The regional difference between maximum and minimum is 1.29 times greater than that of average. The difference between the year with the most rainfall and the youngest is 146 mm, and that in the longest year is almost three times that of the youngest. Third, the most obvious vertical climate change zone is the mountainous area surrounded by mountains on three sides. Especially in the mountainous areas of western Hunan and southern Hunan.

sunshine, air temperature and accumulated temperature

The annual sunshine hours in the province are 13-18 hours, with Dongting Lake as the largest, with Yueyang reaching 184 hours, the southern rivers only reaching 136 hours, and Changsha, Pingjiang and Changde having more than 17 hours throughout the year. The sunshine hours in Anhua and Southwest Passage are 14 hours. As far as the annual average is concerned, the sunshine percentage can reach more than 4% in the lake area, and it is between 3-4% in other places. Seasonally, the sunshine rate in summer and autumn is greater than that in spring and winter. The sunshine hours in July and August are more than 2 hours, and the sunshine rate is more than 5%. The sunshine hours in February are less than 8 hours in most areas. Generally speaking, the sunshine hours are more in the eastern half and less in the western half. The annual solar radiation in the whole province is 86-19 kcal/cm2, which is a province with relatively sufficient light energy in the same latitude. The annual total radiation is the largest in Dongting Lake area, followed by southern Hunan and central Hunan, and the smallest in western Hunan. The regional distribution of solar radiation and sunshine hours is basically the same.

Hunan is rich in heat. The annual temperature is high, the summer heat period is long, the accumulated temperature is high, and the growing season is long. This province is located in the subtropical zone, with a large incident angle of the sun, so it has strong radiation and high temperature. The annual average temperature is between 16-18℃, which gradually decreases from southeast to northwest. Generally, in the area south of Liling, Hengyang and Dong 'an, the annual average temperature is above 17℃, Leiyang, Yizhang, Xintian and Daoxian counties are above 18℃, the north of this line is below 17℃, and Longshan and Huayuan in northwest Hunan are below 16.5℃.

Hunan is controlled by the winter wind in winter, and the landform features that it is surrounded by mountains on three sides in the southeast and open to the north are conducive to the long drive of cold air, so the average temperature in January is mostly between 4-7℃, the general trend of temperature distribution is high in the south and low in the north, and the isotherm is roughly east-west. The extreme minimum temperature is roughly divided by Liuyang, Zhuzhou, Xiangxiang, Qidong, Dongkou and Huitong. The absolute lowest temperature north of this line is generally below -9℃, and there was a low temperature of -18℃ in Linxiang. The absolute lowest temperature south of this line is generally above -9℃. Under the control of subtropical high, the temperature is high in summer. The average temperature in July is above 27℃, with the highest in the lower and middle reaches of Xiangjiang River, and the average temperature in July is above 29℃. The isotherm shows a north-south trend. There was an extreme maximum temperature of 43℃ in Changsha.

Hunan is rich in accumulated temperature, and its heat conditions are richer than those in Hubei, Yunnan, Guizhou, Sichuan, Jiangsu and Zhejiang provinces, and about the same as those in Jiangxi. The accumulated temperature distribution in eastern Hunan is higher than that in northwestern Hunan. The active accumulated temperature ≥1℃ in Xiangjiang River valley south of Liling, Zhuzhou, Hengshan and Dongan counties is mostly above 55℃. The basic situation of accumulated temperature in this province is as follows.

The frost-free period in Hunan is as long as 26-31 days, and most areas are between 28-3 days.

rainfall

The average annual rainfall in Hunan is between 1,2 and 1,7 mm, which is abundant and one of the provinces with more rainfall in China. The guarantee rate of precipitation over 1mm is above 9%; The guarantee rate of greater than 12mm is 7-8%. Rainfall has the following characteristics. First, the regional distribution of rainfall has the characteristics of three more and three less.

The three rainy areas are:

1. The rainy area at the northern end of Xuefeng Mountain. With Anhua as the center, the average precipitation for many years is more than 17 mm, and its surrounding area is also more than 15 mm.

2. The rainy area in the southeast of Hunan, with Guidong as the center, has a rainfall of more than 16 mm and its periphery is also more than 15 mm..

3. Mufu and Jiulingshan rainy areas in eastern Hunan. With Liuyang as the center, the rainfall is more than 15mm. In addition, the rainfall in Jiuyi Mountain in southern Hunan and the upper reaches of Lishui River in northwest Hunan is also quite abundant.

The three rainless areas are:

1. Dongting Lake area in the north of Hunan, with an average annual precipitation of about 1,3 mm, while that in Nanxian County is only 1,23 mm..

2. The dry corridor of Hengshao has an annual precipitation of about 13mm.

3. The annual precipitation in Xinhuang, Zhijiang and Huitong, the southwest border of Hunan, is about 13mm, and Xinhuang is only 1163mm. Second, in the seasonal distribution of rainfall, it has the characteristics of rainy in spring and summer and less rain in autumn and winter. The change of precipitation in this province during the year is roughly consistent with the advance and retreat of the ocean air mass. The rainfall is concentrated in spring and summer, accounting for 7% of the total annual rainfall, and only accounts for 3% in autumn and winter. But spring is more than summer and autumn is more than winter. Thirdly, in the variability of rainfall, the interannual variability is less than the seasonal variability. The annual variation rate of precipitation in this province is small, ranging from 12% to 18%. The seasonal variability of precipitation is large, and it is the largest in summer, generally 4-6%, and it is greater in July than in June. The annual rainfall variability in July is generally 5-6%.