How are the seasons distinguished?

Some are mainly astronomical factors, and some are mainly weather and climate characteristics. Different methods have different seasonal cycles.

Astronomical season

Seasons divided according to astronomical factors. Because the earth's axis of rotation is inclined to its orbital plane around the sun (i.e. ecliptic plane), the variation of solar radiation on the earth's surface circulates regularly once a year. Every year in the same month, all localities generally have their own climate characteristics. In temperate regions, the hottest period receiving the most solar radiation is usually called summer, the coldest period receiving the least solar radiation is called winter, and the transitional period between them is called spring and autumn. For example, in temperate regions of the northern hemisphere, March-May is spring, June-August is summer, September 9- 10/October is autumn165438+1October, and 12- February is winter. In temperate regions of the southern hemisphere, June-August is winter, 65438+2- February is summer, March-May is autumn, and September 9- 10/October is spring. In ancient China, beginning of spring, Changchun, beginning of autumn and beginning of winter started the four seasons, while many countries in Europe and North America started the four seasons with the vernal equinox, summer solstice, autumn equinox and winter solstice. Although the astronomical season has climatic significance, it does not consider geographical and weather factors.

Climatic season

Dividing seasons according to the distribution of climatic factors. The climate season in China was first studied by Zhang (1934). In his article The Distribution of Four Seasons in China, he proposed that the average temperature in five days in winter is below 10C, above 22C in summer, and between 10 ~ 22c in spring and autumn, and drew the length of four seasons in different places. Because the temperature above 10C is suitable for the growth of most crops, and the time above 10C in a year has great influence on agricultural production, it is of great practical significance to divide the seasons in this way.

In addition to temperate seasons, other climatic zones often use other climatic factors to divide climatic seasons because of their special climate. In tropical and sub-tropical areas, the annual change of temperature is small, and the seasons are often divided by the change of precipitation or wind direction, so there are dry seasons and rainy seasons; Northeast trade wind season and southwest trade wind season, etc. This method of dividing seasons is particularly common in the South Asian subcontinent. In most parts of North Africa, a year is divided into three seasons: cool season, hot season and rainy season. Near the polar regions, it is divided into two seasons, namely, summer with eternal days and winter with long nights.

On the plateau of Qinghai-Tibet, the winter is dry and windy, and the summer is rainy for more than half a year, so the whole year can be roughly divided into two seasons: the wind season (dry season) and the rainy season. For other areas with different underlying surfaces, such as oceans and inland areas, forests and grasslands, due to different climates, different criteria for seasonal division can be adopted to meet the needs of local production and life.

The above-mentioned seasonal division methods of natural weather seasons do not consider weather factors, so most of them are not suitable for studying the interannual variation of seasons. In the 1920s, Soviet climatologist Mulitanovsky first put forward the concept of natural weather season. He divided the seasons according to the characteristics of the weather process that formed the climate, and divided the European part of the Soviet Union into five seasons: spring, summer, autumn, previous winter and winter. Later, C.T. Pagava divided summer into early summer and midsummer. In 1950s, Yang Primary School in China studied the natural weather season in East Asia according to the circulation pattern of 500 hectopascals. The division of natural weather seasons is one of the research contents of weather climatology, which is of great significance to long-term weather forecast. However, due to the complexity of the weather process, there is still a lack of objective or unified standards for the division of natural weather seasons. Therefore, the start and end dates of the season are not easy to determine. At the same time, for a natural weather season, the weather performance is different because of different locations and different weather systems. Therefore, the concept of natural weather season is still immature and needs further study.