Why do the temperatures in the four seasons change?
The four seasons on the earth are first manifested as an astronomical phenomenon, which is not only the periodic change of temperature, but also the periodic change of the length of day and night and the height of the sun. Of course, the change of the length of day and night and the height of the sun at noon determines the change of temperature. The gradual changes of the four seasons are not uniform all over the world. It is summer in the northern hemisphere and winter in the southern hemisphere. The northern hemisphere changes from warm to cold, and the southern hemisphere changes from cold to hot.
Now, analyze the periodic changes of the length of day and night and the height of the sun in different seasons.
From vernal equinox to summer solstice to autumnal equinox, the northern hemisphere is in summer and the southern hemisphere is in winter. During this period, the northern hemisphere has long days and short nights, while the southern hemisphere has short days and long nights. The north pole is in the extreme day and the south pole is in the extreme night; The height of the sun north of the Tropic of Cancer is always greater than the average, while the height of the sun south of the Tropic of Cancer is less than the average. North of the Tropic of Cancer, the sun rises on the northeast horizon and sets on the northwest horizon. On the vernal equinox, the sun all over the world rises in the east and sets in the west.
From the autumnal equinox to the winter solstice and then to the vernal equinox, the northern hemisphere is in the winter half year and the southern hemisphere is in the summer half year. During this period, the length of day and night, the extreme value of day and night and the height of the sun in the northern and southern hemispheres are contrary to the above situation. North of the Tropic of Cancer, the sun rises from the horizon in the southeast and sets on the horizon in the southwest.
From summer solstice to winter solstice, from summer to winter in the northern hemisphere and from winter to summer in the southern hemisphere. During this period, the days in the northern hemisphere are getting shorter and shorter, the nights are getting longer and longer, and the extreme day area is getting smaller and smaller. In the southern hemisphere, the days are getting longer and longer, the nights are getting shorter and shorter, and the polar night zone is getting smaller and smaller. The height of the sun has been declining in the north of the Tropic of Cancer and rising in the south. North of the Tropic of Cancer, the rising direction of the sun changes from northeast to southeast, and the setting direction changes from northwest to southwest. The autumnal equinox rises due east and lands due west.
From the solstice of winter to the solstice of summer, from winter to summer in the northern hemisphere and from summer to winter in the southern hemisphere. The changes of the length of day and night, the extreme value of day and night, and the height of the sun in the northern and southern hemispheres are contrary to the above situation. North of the Tropic of Cancer, the rising direction of the sun changes from southeast to northeast, and the setting direction changes from southwest to northwest.
From the winter solstice to the vernal equinox, from the summer solstice to the autumnal equinox, the number of days around the world is close to the average (12 hour), and the range of extreme days and nights is gradually shrinking. The height of the sun north of the Tropic of Cancer and south of the Tropic of Capricorn is close to the average. North of the Tropic of Cancer, the rising direction of the sun gradually approaches due east, and the setting direction approaches due west.
From vernal equinox to summer solstice, from autumnal equinox to winter solstice, the length of day and night around the world is changing to an extreme value, and the range of extreme day and night is gradually expanding. The height of the sun north of the Tropic of Cancer and south of the Tropic of Capricorn also tends to be extreme. North of the Tropic of Cancer, the rising and setting directions of the sun move to the northeast, southeast, northwest and southwest respectively.
Because the variation of the length of day and night and the height of the sun between the Tropic of Cancer is complicated and occupies a large space, we have not fully explained it, and it is not difficult for readers to sum up the rules themselves. When analyzing, it is best to divide it into several stages. For example, in the northern hemisphere, it can be seen as a stage from vernal equinox to direct sunlight, followed by the second stage from summer solstice, the third stage from summer solstice to direct sunlight, and the next stage from winter solstice to spring, which is the last stage, and the sun has completed a regression movement. The length of day and night at each stage, the height of the sun, the rising and falling direction of the sun and the direction of the sun at noon (for example, during the summer solstice in the northern hemisphere, the sun is located in the north of the zenith at noon, and in the winter solstice, it is located in the south of the zenith), and so on.
2. Division of four seasons
The four seasons are divided according to the length of day and night and the change of sun height. The division of the four seasons is based on the apparent position of the sun on the ecliptic and bounded by the bisector, the second solstice or the fourth day. However, the dividing points used by eastern and western countries to divide the four seasons are not exactly the same. The traditional method of dividing the four seasons in China emphasizes the astronomical significance of the four seasons, with four of the 24 solar terms as the starting point of the four seasons and bisection and two to one as the midpoint. For example, in spring, the starting point is beginning of spring, the solar line is 3 15, the spring is divided into the midpoint, the long summer is the end point, the solar line becomes 45, and the sun runs 90 on the ecliptic.
The division of four seasons in the west emphasizes the climatic significance of the four seasons, taking the bisector of the sun as the starting point of the four seasons, such as spring as the starting point and summer solstice as the end point. These four seasons are one and a half months later than those divided by China.
Astronomically speaking, it is more scientific for China to divide the four seasons with four stands.
At the equinox, the length of day and night and the height of the sun around the world are equal to the annual average, which has the typical characteristics of transition from maximum (or minimum) to minimum (or maximum). Therefore, it is very reasonable to regard the vernal equinox as the midpoint of spring and the autumnal equinox as the midpoint of autumn. In summer, the day is the longest and the night is the shortest. On the day of summer solstice, the height of the sun is the highest and the surface gets the most solar energy. Therefore, it is very reasonable to take the summer solstice as the midpoint of summer; Similarly, the winter solstice as the midpoint of winter is also very scientific.
However, from the actual climate, the summer solstice is not the hottest time, and the winter solstice is not the coldest time, so the extreme temperature should be delayed by 1 ~ 2 months respectively. There is a saying in China that "heat is in dogs" and cold is in Sanjiu. Therefore, taking summer solstice and winter solstice as the start dates of summer and winter respectively can better correspond to the actual climate. Therefore, the division of four seasons in the west can better reflect the actual climate significance.
Whether it is the division of four seasons with astronomical significance in China or the division of four seasons with climatic significance in the west, it is an astronomical division method. This is because dichotomy, dichotomy and quartering all have exact meanings in astronomy. They all divide the year into four roughly equal seasons, each lasting for three months, and the sun runs 90 degrees on the ecliptic. None of them can reflect the actual situation of the local climate. The seasons divided in this way are the four seasons in astronomy.
The astronomical season is hemispherically unified. In the hemisphere, every season has a unified starting and ending time. In the hemisphere, there are four seasons everywhere, and each season is of equal length.
In order to accurately reflect the actual climate situation in various places, climate methods are often used to divide the four seasons, such as using the average climate temperature to divide the four seasons. It is also stipulated that the period when the average temperature is greater than or equal to 22℃ is summer, the period when it is less than or equal to 10℃ is winter, and the period between 10℃ and 22℃ is spring or autumn. According to this standard, the seasons in mid-latitude areas are consistent with the climate, while the temperature changes in low-latitude areas and near the polar regions are not obvious. At the same time, in mid-latitude areas, the length of each season is different. This is the four seasons. For example, Beijing has 55 days in spring, 0/03 days in summer, 50 days in autumn and 0/57 days in winter.
Astronomical seasons have theoretical significance, while climatic seasons have practical value. Astronomical seasons are the basis for dividing climatic seasons. The astronomical season is hemispherically unified. It is summer in the northern hemisphere and winter in the southern hemisphere; The climate of the four seasons is unified in some areas (mid-latitude areas). The division of astronomical seasons depends on the change of astronomical phenomena, and the division of climatic seasons depends on the change of temperature. No matter where in any hemisphere, there are astronomical seasons of equal length; In the same place, the time of four climatic seasons is not necessarily equal. This is the main difference between astronomical season and climate season.