Why is the weather in Changsha so changeable?

The spring and autumn in Hunan is very short, and the summer is sultry and rainy, and the highest temperature is often around 40℃. In winter, the weather is cold, rainy and occasionally snows, and the lowest temperature is often below 0℃. It's a hot summer day, and the highest temperature in Changsha during the day is even higher than that in Guangzhou in Lingnan! Why is the climate in Hunan so extreme? This has a lot to do with the topography of Hunan, which is surrounded by mountains on three sides in the southeast and west, and only the north is connected with Dongting Lake. In winter, the Siberian cold air moves south, sweeping away the water vapor in Dongting Lake, which can't be dispersed when it is blocked by mountains, and it rains heavily and can't be opened for months. In Lingnan area, because of the blocking of Nanling Mountains, it is difficult for cold air to blow in, and the temperature in winter is relatively high. However, in summer, humid air currents and typhoons in the south are also difficult to affect Hunan because of the blocking of mountains. So in summer, Wan Li is clear, with little rain but high humidity, and it is sultry. It is called "sauna day", just like a sauna.