Why is the weather in Hunan hot and cold?

Climatic 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 30 08' north latitude, living in the southeast of Eurasia, facing the Pacific Ocean, and the border with Southeast Asia is 400 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 65438+10, the total radiation accounts for 70-76% of the total annual radiation, and the precipitation accounts for 68-84% of the total annual precipitation. Second, the climate changes greatly during and between years. It is hot in Leng Xia in winter, changeable in spring, sudden drop in autumn, rainy in spring and summer, and drought in autumn and winter. The climate changes greatly from year to year. The regional difference between the maximum value and the minimum value is 1.29 times larger than the average value, and the difference between the year with the most rainfall and the youngest year is 1.460 mm, almost three times that of the youngest year. Third, the most obvious vertical climate change zone is the mountainous area surrounded by mountains on three sides. Especially in the mountainous area of southern Hunan.

Sunshine, temperature and accumulated temperature

The annual sunshine hours in the province are 1.300- 1.800 hours, with Dongting Lake being the most, Yueyang reaching 1.840 hours, southern rivers only reaching 1.360 hours, and Changsha, Pingjiang and Changde exceeding1.70 hours. The sunshine hours in Anhua and Southwest Passage are 1400 hours. In terms of annual average, the sunshine percentage in the lake area can reach more than 40%, and it is between 30-40% in other places. Seasonally, the sunshine rate in summer and autumn is greater than that in spring and winter. In July and August, the sunshine hours are over 200 hours, and the sunshine rate is over 50%. The sunshine hours in most areas in February were less than 80 hours. Generally speaking, sunshine hours are more in the eastern half and less in the western half. The solar radiation in the whole province is 86- 109 kcal/cm2. In, it is a province with sufficient light energy at the same latitude. The total annual 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 growth period is long. The 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℃, and gradually decreases from southeast to northwest. Generally, it is in the south of Liling, Hengyang and Dong 'an, and 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 at/kloc-0.

The winter in Hunan is controlled by the winter wind, surrounded by mountains in the southeast and northwest, and open to the north, which is conducive to the long-term driving of cold air. Therefore, the average temperature of 1 month 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 bounded by Liuyang, Zhuzhou, Xiangxiang, Qidong, Dongkou and Huitong. The absolute minimum temperature north of this line is generally below -9℃, and the low temperature of-18℃ appears in Linxiang. The absolute minimum temperature south of this line is generally above -9℃. Controlled by subtropical high, the temperature is high in summer. The average temperature in July is above 27℃, the highest in the middle and lower reaches of Xiangjiang River, and the average temperature in July is above 29℃. The isotherm shows a north-south trend. The extreme maximum temperature of 43℃ appeared in Changsha.

Hunan is rich in accumulated temperature, and its heat condition is richer than that of Hubei, Yunnan, Guizhou, Sichuan, Jiangsu and Zhejiang, and similar to that of Jiangxi. The accumulated temperature distribution in eastern Hunan is higher than that in northwestern Hunan. The active accumulated temperature in the Xiangjiang River basin ≥ 10℃ south of Liling, Zhuzhou, Hengshan and Dongan counties is mostly above 5500℃. The basic situation of accumulated temperature in this province is as follows.

The frost-free period in Hunan is as long as 260-3 10 days, and most areas are between 280-300 days.

rain

The average annual precipitation in Hunan is between 1200- 1700 mm, which is abundant and one of the provinces with more rainfall in China. /kloc-more than 0/000 mm precipitation guarantee rate of more than 90%; 70-80% guarantee rate is greater than 1200mm. 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, rainy area at the northern end of Xuefeng Mountain. With Anhua as the center, the average precipitation for many years is above 1700 mm, and its surrounding area is also above1500 mm.

2. The rainy area in the southeast of Hunan, with Guidong as the center, the rainfall is above 1600 mm, and its periphery is also above 1500 mm. ..

3. The rainy areas of the shogunate and Jiuling Mountain in eastern Hunan. With Liuyang as the center, the rainfall exceeds 1500mm. 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 Province, the average annual precipitation is about 1300mm, while that in Nanxian County is only 1230mm.

2. Arid corridor in Shao Heng, with annual precipitation of about 1300mm.

3. The annual precipitation of Xinhuang, Zhijiang and Huitong in the southwest border of Hunan is about 1300mm, and Xinhuang is only 1 163mm. Second, in the seasonal distribution of rainfall, it has the characteristics of rainy in spring and summer and little rain in autumn and winter. The change of annual precipitation in this province is roughly consistent with the advance and retreat of marine air mass. Rainfall is concentrated in spring and summer, accounting for 70% of the total annual rainfall, and only 30% in autumn and winter. But spring is more than summer, and autumn is more than winter. Third, in the change of rainfall, the interannual change is less than the seasonal change. The annual variation rate of precipitation in this province is small, ranging from12 to18%. The seasonal variation of precipitation is great, which is the largest in summer, generally 40-60%, and July is greater than June. The annual rainfall change rate in July is generally 50-60%.