The main reasons for the climate difference between Harbin and Hainan are

Compared with other capital cities in China, Harbin has the highest latitude and the lowest temperature in winter. Harbin is controlled by polar continental air mass in winter and mainly influenced by subtropical ocean air mass in summer. Spring and autumn are the seasons in which winter wind and summer monsoon alternate. Its climate type is temperate continental monsoon climate. The climate of Harbin, like other regions, is formed under the long-term interaction of solar radiation, atmospheric circulation, topographic conditions and human activities. 1. Solar radiation is the most important source of heat energy in the earth's atmosphere and the most basic and important factor affecting climate change. The climate difference between Harbin and different seasons is directly related to the distribution of solar radiation and its change with time. Harbin is located between the Tropic of Cancer and the Arctic Circle, and the variation of solar altitude angle in a year and solar radiation in related seasons is very large. Harbin winter solstice sunrise at 7: 37, sunset 16: 20, the day length is only 8 hours and 44 minutes, and the noon sun altitude angle is 2015'. On the summer solstice, the sun rises at 4: 19 and sets at 19: 53. One day is 1 5 hours and 34 minutes, and the height of the sun at noon is 67 45'. There is a great difference in the length of days between winter and summer. The noon sun height from winter solstice is smaller than that from summer solstice 1/3. According to the data of Harbin Solar Radiation Observatory (45 41'n,126 37' e)1961~1986, the annual average total solar radiation in Harbin is1/. Monthly and annual direct radiation, scattered radiation and total radiation (Table 3- 1). The maximum value of total radiation in Harbin appeared in June, reaching 16.8 kcal /cm2, and the minimum value appeared in June, 12, which was 2.5 kcal /cm2, with an annual variation of 14.3 kcal /cm2 (Table 3-2). The daily variation of total radiation increased significantly from February to March, with an increase rate of 37 kcal/cm2. It dropped significantly from July to September, with a monthly drop of 2.6-2.7 kcal/cm2 (Table 3-3). The direct radiation and scattered radiation in Harbin's total solar radiation also change obviously with the seasons. Due to the different properties of atmospheric circulation and underlying surface in each month of the year, the composition of the two components is obviously different in different months, which mainly depends on the difference of cloud cover and atmospheric transparency in each month. 2. Atmospheric circulation The climate in Harbin is not only influenced by local solar radiation and geographical conditions, but also restricted by hydrothermal conditions in atmospheric circulation. Atmospheric circulation promotes the heat exchange and water exchange between high and low latitudes and between land and sea, and the seasonal change of atmospheric circulation directly affects the seasonal change of Harbin climate. In winter, the westerly belt in the northern hemisphere is a three-wave type. There is an average high-pressure ridge (Ural Ridge) in the eastern part of the Ural Mountains in Asia, and an average low-pressure trough (East Asia trough) in the eastern part of the Asian continent. Under the circulation background of "West Ridge East Trough", Harbin is often controlled by the northwest airflow behind the front ridge trough. At the same time, the Mongolian cold high and Aleutian low in the surface layer of Asia have been established stably, and Harbin is often at the forefront of Mongolian cold high, so the northwest monsoon prevails. The whole winter in Harbin is mainly influenced by this high and low circulation background, which leads to a cold and dry climate. In summer, the westerlies in the northern hemisphere change from three waves in winter to four waves in summer. At this time, the average coastal area at middle and high latitudes in East Asia is a weak ridge, and the northwest is a wide trough of low pressure. Accordingly, the Asian continent near the surface layer is a thermal low pressure area on average, blocking high pressure occurs frequently in the northern part of East Asia coast, and the continental low pressure activity north of 40 north latitude is also frequent. At this time, Harbin is often controlled by the northeast low pressure and influenced by the Pacific ridge, and the southeast monsoon is strengthened. In summer, the subtropical front moves northward, and cold and warm air often passes through Sui at 45 north latitude. Because the western Pacific ridge is located in the north-west direction, it is beneficial for cyclones in Jianghuai, North China and Mongolia to move eastward or northward along the upper front and the airflow on the west side of the Pacific ridge, and Harbin area enters the main precipitation season at this time. Spring and autumn are transitional seasons. In these two seasons, Harbin is mainly influenced by the denatured polar continental air mass, but the denatured polar continental air mass is weakening in spring, and the opposite is true in autumn. The situation of high altitude in spring is similar to that in winter, but the long wavelength becomes shorter and begins to transition to summer circulation type. The average position of the East Asian trough moved eastward slightly, and its strength weakened, widened and flattened. The westerly winds prevail in high latitudes in Asia, and the front area is strengthened. In the near-surface layer, the Mongolian high often remains in the Bohai Sea and the Yellow Sea after moving eastward into the sea. The mainland began to warm, and the low pressure often appeared in Mongolia and deepened eastward along the high-altitude front, confronting the high pressure in the Bohai Sea and the Yellow Sea, forming a situation of high in the south and low in the north, and the pressure gradient was strengthened. In this case, Harbin is often southerly, with little rain and drought. In autumn, the polar front area strengthens and gradually moves southward, and the cold air force increases. The cold high pressure near the mainland surface began to move in Mongolia, while Harbin was often affected by the northeast low pressure, and sometimes there was heavy precipitation. When the cold air is further strengthened, Harbin area is often under the control of a single cold high pressure, the temperature drops rapidly, and the weather situation changes rapidly to winter type.