It is the part of the atmosphere or space that an observer can see on the surface of a celestial body. On the earth, the clear sky is blue. At sunrise and sunset, the sky turns red. Visibility in clear skies is usually described by visual depth.
When sunlight enters the atmosphere, colored light with the same wavelength as red light has strong transmittance and can penetrate the atmosphere and reach the ground. However, violet, blue and cyan light with shorter wavelength will easily scatter when it touches atmospheric molecules, ice crystals and water droplets. Scattered purple, blue and cyan light fills the sky, making it look blue!
People can observe meteorological or astronomical phenomena in the sky, so as to know the change of weather, the passage of time or their position. You can know the time of day at sunrise and sunset, and the moon's profit and loss for a month at night. The big dipper can indicate the north. The thickness and shape of clouds can judge whether it will rain or not.
You can enjoy many beautiful phenomena in the sky, such as rainbow, aurora and meteor shower. Birds can fly in the sky.
Simply put, you go outside and look up. But the sky is as blue as a canvas, occasionally mixed with clouds! !
Due to the increasing use of fossil fuels such as oil, especially fuels such as coal that will release sulfur dioxide after burning, the visibility of the sky has gradually decreased since 1973, except in Europe.
The fundamental reason for the seasonal change is that the earth's rotation axis is not perpendicular to its orbital plane (ecliptic plane), and it forms an inclination angle (ecliptic angle) of 23 degrees and 26 minutes with the vertical of the orbital plane. If the northern hemisphere is the head of the earth, then the earth seems to be tilted 23.26 degrees around the sun from head to toe (this angle will change slowly and slightly), and the tilt direction remains unchanged, so that as the earth rotates around the sun, the North Pole will gradually face the sun or space. In winter in the northern hemisphere, on the other hand, when the earth revolves around the sun, the northern hemisphere gradually faces space instead of the sun, and the North Pole does not bask in the sun for a long time, and the cold air increases to form winter in the northern hemisphere. The season in the southern hemisphere is opposite to that in the northern hemisphere, because the south pole of the southern hemisphere faces space when there is a large area of sunlight in the northern hemisphere in summer, and the temperature drop of the sun is completely ignored for a long time, forming the southern hemisphere into winter. Spring and autumn are transitional seasons. When the direct point of the sun is close to the equator, although the northern hemisphere and the southern hemisphere gradually face space instead of the sun, the earth faces the sun like a "side", and the sunshine areas in the northern and southern hemispheres are the same, so the climate is neither cold nor hot.
China people believe that the four seasons have different characteristics, namely "spring", "summer", "autumn" and "winter storage". That is, everything grows in spring, grows in summer, harvests in autumn (mature) and hides in winter (animals hibernate and plants shed leaves).
Sunlight forms seven colors through light scattering: red, orange, yellow, green, blue, indigo and purple. Red light is the strongest, orange, yellow and green are also stronger, and the weakest are blue, indigo and purple. When sunlight passes through the thick atmosphere, red light runs fastest and passes through it at once; Followed by orange, yellow and green light; Most of the blue and indigo light is blocked by the atmosphere. They are pushed around by dust and water droplets floating in the atmosphere and reflected back and forth (eventually entering our eyes). As a result, the sky turned blue. (i.e. "Rayleigh scattering" effect)
In spring, the northern hemisphere of the earth began to tilt towards the sun, receiving more and more direct sunlight, so the temperature began to rise. As the ice and snow melt, the water level of the river rises. In spring, plants begin to sprout and grow, and many flowers bloom. Hibernating animals wake up, many animals with eggs for the winter hatch, and birds begin to migrate, leaving the wintering ground for breeding grounds. Many animals are in estrus during this period, so China also calls spring the season of "the revival of everything".
Spring is the wettest season in many parts of the earth because hot air begins to move northward, while cold air often stays, and the temperatures of land, water and air rise at different speeds. There are famous plum rains in southern China, strong storms in Europe and tornadoes in North America.
The angle between the equator and the orbit of the earth is the fundamental reason for the seasonal variation. In spring, the direct point of the sun gradually moves from the tropic of Capricorn to the north, crosses the equator after the vernal equinox, and the sun shines directly into the northern hemisphere. In spring, the distance between the earth and the sun is getting farther and farther. Every year around 65438+10.3, the earth is closest to the sun. Seen from the ecliptic plane, the sun is on the background of Aquarius, Pisces and Aries.
In most areas, summer is the season with hot climate and possible water shortage, but in monsoon climate areas such as China, summer is the season with the most concentrated precipitation.
The high temperature in summer in the northern hemisphere is the most remarkable climate feature. Due to the difference of region and dry and wet environment, it will produce dry and hot or wet and rainy climate. In China's coastal areas, the southwest monsoon will generally be introduced during the inland heating in May, and a low pressure will be formed in the southwest. Generally, the southwest monsoon will reach China's coastal areas around May, and the northeast monsoon will be blocked by the southwest monsoon, so it will not come to China's coastal cities all summer. The northeast monsoon along the coast of China will officially end, and the first wave will not come until late August to mid-September. China coastal cities usually define the arrival of summer by the arrival of the first southwest monsoon.
The cold air in the north meets the warm air brought by the southwest monsoon in the south, resulting in a trough of low pressure in the South China Sea. Moreover, affected by cold and warm air, there will be a period of heavy rain lasting for many days in Hong Kong and other places. This is the beginning of the pre-flood season in South China in Hong Kong and other places. It was not until July that the subtropical high moved northward and pushed the cold air and low pressure trough into the inland cities of China that Hong Kong ended the pre-flood season in South China.
However, the Yellow River Basin in Chinese mainland was affected by the northward movement of the low pressure trough in South China, and it was generally rainy in early July. This is the beginning of the rainy season, which will end in mid-July. The second flood season in Hong Kong usually begins in August, because at this time, tropical cyclones will enter the South China Sea westward under the influence of subtropical high, continue to affect Hong Kong or the monsoon trough will affect Hong Kong northward, causing heavy rain in Hong Kong. However, the rainstorm in the late flood season in South China is usually paroxysmal, and the rainfall is generally larger than that in June, but the persistence is poor. Generally speaking, the late flood season in South China will generally last until the arrival of the first northeast monsoon in September and come to an end. Due to the southward movement of the westerly trough, there is no tropical cyclone landing under South China, and the late flood season in South China generally ends from mid-September to1early October.
Because most areas will be affected by low pressure, the climate is relatively stable. However, the ocean near the equator will form a typhoon, which will easily cause damage to southern China, especially Fujian, Guangdong and Hainan.
Autumn is the harvest season, and the fruits of many plants mature in autumn. In the subtropical region of the northern hemisphere, the temperature in autumn is obviously lower than that in summer. As the temperature drops, the leaves of many deciduous perennial plants will gradually change color, wither and fall off, leaving only branches for winter. Panicum miliaceum will come to the end of his life, withering and dying.
The temperature will gradually drop in autumn, but it is generally slower than in winter. Due to the difference of dry and wet conditions, there will be cold and rainy weather conditions or dry and cool weather conditions in different regions. In the cold late autumn, due to the large temperature difference between day and night, the water vapor transpiration during the day will condense at night, either dew or frost.
In autumn, the direct point of the sun gradually moves south from the northern hemisphere, passes through the equator after the autumnal equinox, and the sun directly shines in the southern hemisphere. Seen from the northern hemisphere, the angle of the sun gradually becomes lower, and the gap between day and night becomes smaller. At the autumnal equinox, the length of day and night is equal. In autumn, the distance between the earth and the sun is from far to near. Seen from the ecliptic plane, the sun is on the background of Leo, Virgo and Libra.
In the northern hemisphere, winter is the coldest season, and even the temperature in temperate zone may drop below 0℃.
Many places will experience snowfall in winter, and the snow and ice in some places will not melt until spring.
In winter, the direct point of the sun moves south to the tropic of Capricorn, and then turns back to the north.