Relevant information of temperate desert climate

Temperate desert is mainly distributed in the subtropical high control area near the Tropic of Cancer. Trade winds between north and south latitudes 15-35 degrees. The air pressure in these places is high and the weather is stable. The wind always blows from the land to the sea, but the humid air from the sea can't enter the land, so the rainfall is very small and very dry. The rocks on the ground are weathered to form tiny sand grains, which float and accumulate with the wind, forming sand dunes, which are widely distributed and become vast deserts. In some places, the weathering speed of rocks is slow, forming large gravel, which is desert.

Most of these deserts live in the depths of the mainland, far from the sea, blocked by mountains and terrain. It is difficult to reach the humid ocean airflow, and the climate is very dry, forming a desert. Such as Karakum Desert in Central Asia and Kum Desert in Kutzier, Great Gobi Desert in Mongolia, Taklimakan Desert in China and Great Desert in the western United States. Arabian Desert in West Asia, thar desert in Northwest India, etc. In the southern hemisphere, there are Dasha Desert, Gibson Desert, Grand Victoria Desert, Atacama Desert in northern Chile, kalahari desert and namib desert in southern Africa, Qom Desert in Cortesi (Uzbekistan) in Central Asia, Karakum Desert (Turkmenistan) and the desert in northwest China.

China desert is distributed in different natural zones from west to east. Due to different natural conditions, the characteristics of each desert are obvious, that is to say, from west to east, quicksand gradually decreases and fixed and semi-fixed sand dunes gradually increase. The deserts in China mainly include Taklimakan Desert, Gurbantunggut Desert, Badain Jaran Desert, Tengger Desert, Qaidam Desert and smaller Kumtag Desert, Mu Us Sandy Land, Hunshandake Sandy Land, Horqin Sandy Land and Kubuqi Desert, with a total area of about 600,000 square kilometers, of which Taklimakan Desert accounts for half of the desert area in China, accounting for 337,600 square kilometers. Desert is the product of dry climate, and dryness and lack of rain are the necessary conditions for desert formation. Seen from the whole earth, the formation of dry climate zone (arid zone) is mainly related to latitude, atmospheric circulation and other factors. Between latitude 15 ~ 35, it is controlled by subtropical high pressure area (also known as reflux high pressure area), and it is an area that is hit by trade winds all the year round. The air in the high pressure area has a sinking effect. When the air sinks, it forms adiabatic heating, which reduces the relative humidity and makes the air very dry. The trade wind is a stable wind direction blowing from the subtropical high pressure belt to the equatorial low pressure belt, and it keeps heating up in the process of blowing to the equator; The hotter the air, the more water is consumed, which forms a very dry dry wind. In this way, in the subtropical high zone control area, the atmosphere is very stable, the humidity is low, and it is cloudy and rainy locally, which is an arid area with little rainfall on the earth. Most of the great deserts in the world are distributed here, such as the Sahara Desert in North Africa, the Arabian Desert in Southwest Asia and the Atacama Desert in South America. Therefore, it is called "returning to the desert".

China Desert is located in the temperate zone between 35 ~ 50 north latitude and 75 ~125 east longitude. According to their latitude, they are not in desert in the world, but around 0/5 north latitude and 20 east longitude. China and South China, with the same latitude as desert in the world, are not only desert-free, but also warm and humid all the year round. Whether it is plain or mountain, it is lush everywhere.

China is located in the southeast of Eurasia, the largest continent in the world, close to the Pacific Ocean, the largest ocean in the world. This land-sea distribution and thermal difference between land and sea have great influence on China's climate. In winter, the air on the mainland is colder than the air on the ocean, and it shrinks thicker than the air on the ocean, and the air pressure increases. Especially in Siberia and Mongolia, Russia, which are located in the hinterland of middle and high latitudes, the heat radiated by the sun in winter is very weak, and the night is long, so a lot of heat is lost, so the air is very dry and cold, and a large amount of cold air accumulates to form a strong high pressure area. On the contrary, China's Southern Ocean is a low-pressure area. The air in the high-pressure area continuously flows to the low-pressure area, forming a northerly wind prevailing in winter in China. In summer, the air on the mainland is hotter than that on the ocean, and its expansion is thinner than that on the ocean. The air pressure on the ocean is high, forming a high pressure area, and the air blows from the humid ocean to the mainland, which makes the southerly wind prevail in China. This kind of wind that changes with the seasons is called monsoon.

As China is an area where East Asian monsoon prevails, the water vapor of precipitation is mainly brought by the humid summer monsoon from the southwest Pacific Ocean, the South China Sea, the Bay of Bengal and the Indian Ocean. Therefore, the southeast coast of China and South China, which are located in the subtropical zone, bear the brunt with abundant rainfall, and become humid areas with more rainfall at the same latitude in the world. In the vast northwest and Inner Mongolia, which are located in temperate zones, the summer monsoon has become a spent force there because they live inland and far from the ocean. In addition, on its southern and southeastern edges, there are tall mountain systems such as Tianshan Mountain, Kunlun Mountain, Qinling Mountain, Luliang Mountain, Daxinganling Mountain, etc. These mountain systems were raised by orogeny in the late Tertiary and early Quaternary, especially the huge Qinghai-Tibet Plateau, which became an insurmountable barrier for the summer monsoon. In this way, the humid ocean airflow (southeast monsoon and southwest monsoon) can't blow in, and the source of water vapor is isolated. As a result, in the northwest and Inner Mongolia in summer, the water vapor is poor and the rainfall is scarce. In winter, due to the relatively open terrain in the northwest and northern Inner Mongolia, there is no mountain barrier, and the strong dry and cold airflow from the Mongolia-Siberia high pressure area can flow down directly, resulting in an unusually dry and cold climate.

Due to the above reasons, the vast areas in northwest China and Inner Mongolia are extremely dry all the year round, forming the world's largest temperate inland arid and semi-arid region with a typical continental climate. The climate is dry, precipitation is scarce, and the role of running water is very weak; On the contrary, the activity of wind is very active, especially the bare ground without plant cover formed under the influence of dry climate, which further promotes and strengthens the role of wind and makes it the main force to shape landforms. Under the action of wind, the loose and bare sand surface has a strong wind erosion, and the sand particles are carried by the wind: when the wind weakens or meets obstacles and the wind cannot carry the sand particles forward, the sand particles pile up into sand dunes, which further develop and expand, and finally form a vast desert. On the other hand, the expansion of desert intensifies the formation and expansion of desert climate. Temperate desert climate has many characteristics, such as sunny days, strong sunshine, dryness, hot summer, large temperature difference between day and night, many sandstorms, "dry summer and wet winter" and so on. The key is "doing". Because it is dry, there is less rain, more sunshine and strong sunshine; Because it is dry, there is no cloud in the sky, so it can't stop some sunshine, and there is no water on the ground, so it can't evaporate and cool down. All the heat from the sun on the ground is used to heat the earth and the air, so the temperature in summer is particularly high, and the temperature difference between day and night is particularly large. Because of the dry weather, there are few plants on the ground. When there is wind, it will blow sand.

In the desert area of China, the annual rainfall is mostly below 50- 100 mm, and the least area is only 10-20 mm. Turpan, which is rich in raisins and cantaloupes, has an annual rainfall of only 16.4 mm, and Toksun County has even less rainfall, even 6.9 mm. When it comes to rainy days, Turpan has an average annual rainfall of only/. 1958 August 14 A rain, 36 mm, was the biggest since the founding of the People's Republic of China. In desert areas, sometimes the sky is overcast, the wind roars and there is dazzling lightning. I often see thunderstorms coming, but after waiting for a long time, the clouds have dispersed and the raindrops have not come down yet. In fact, it is raining in the sky, just because the air is too dry, and the raindrops have evaporated by half through the thick dry atmosphere before falling to the ground. This situation is called rain flag in meteorology. In the past, most people didn't understand it. It was called "Devil Rain".

However, due to the extremely high surface temperature in the desert, air convection is extremely strong, and sometimes huge raindrops will be produced in the clouds. These raindrops can't evaporate all the way, but they can still fall to the ground. However, the raindrops in this rain are very thin. A geographer said with a smile that if people can stay between raindrops all the time, they can walk in the rain without getting wet.

Meteorological departments often use relative humidity to measure the amount of water vapor in the air. When the relative humidity is 100%, the air is saturated. The average relative humidity in desert areas is only about 20%, and it is often lower than 10% in the afternoon. Sometimes, in meteorological records, there will be 0% records. That is to say, there is no water vapor in the air, at least the instrument can't detect it. When the relative humidity is very low in summer, if you wash clothes one by one, generally speaking, by the time you wash the third one, the first one is dry and really ready to wear.

At noon in the desert, in China, the record of 70℃ is not uncommon. The highest scale of the ground thermometer in Turpan is 75℃, but the mercury column far exceeds 75℃ several times. 82.3℃ was measured on the surface of sand dunes in the south of Turpan Basin.

The high temperature in the desert during the day is due to strong solar radiation. The annual sunshine time in desert areas exceeds 3,000 hours, which can exceed 3,600 hours in the most places and even reach 4,000 hours in the Sahara desert. In other words, the daily sun exposure time exceeds 10 hour. However, when the sun goes down, there is no cloud in the atmosphere, and there is little water vapor, and the speed of ground cooling and heat dissipation is also very fast, so the daily temperature difference in desert arid areas is particularly large. In summer, the average difference between the highest temperature in the afternoon and the lowest temperature in the morning is about 15℃- 16℃. For example, the average minimum temperature in Turpan in July is 25. 1℃, and the average maximum temperature is 39.9℃, which is 14.8℃. In September, the highest temperature in Turpan in the afternoon averaged 27.5℃, often exceeding 30℃, while the lowest temperature in the morning averaged only 9.3℃, and on some days it could drop to 0℃-5℃. Therefore, Turpan basin is known as "wearing leather clothes early and wearing yarn at noon".

On many Gobi deserts in the world, due to the exposure of the scorching sun, rocks are broken and shooting sounds are often made. Some people call it the sound of the sun This is because the daily temperature difference of desert climate is large, and the stone that is already very cold at night is heated strongly after the sun rises, and the stone expands unevenly inside and outside and breaks. Over time, stones will eventually turn into sand.

According to records, in Qilian Mountain area of China, people used to crush rocks with their hands, and the wind blew sand to extract gold. Under the attack of nature's cold and heat, the hardest granite was finally shattered.

The high temperature on the desert surface makes the temperature difference between the upper and lower atmosphere close to the ground extremely large. However, the temperature distribution of cold air layer and hot air layer can reflect the distant scenery in front of travelers, which is called a mirage in meteorology, commonly known as a mirage. Therefore, travelers in the desert often see a blue lake in front of them. In fact, it is the reflection of the distant sky that makes them happy. On the black asphalt pavement in summer afternoon, we can often see a piece of blue water on the road ahead, but when we look closer, it disappears and moves to the front. This is also a mirage scam on the road.

The four seasons in the desert are also very different from other places at the same latitude. In eastern China, the spring and autumn seasons are very short because of the monsoon continental climate. However, in desert areas, spring and autumn are shorter. Because the desert area is too dry, there is no water regulation. The temperature rises in a straight line in spring and falls in autumn. The sum of spring and autumn is only about two and a half months to three months. Spring and autumn are short, and winter and summer are particularly long, so some people describe it as: "In the arid areas of Central Asia, there are only two seasons in a year: winter in Siberia and summer in the Sahara." There is some truth in this statement. The temperature change of desert climate is the most extreme among all kinds of climate in the world. Mirage is a unique landscape in temperate desert climate zone. Looking at the sea, or sailing on the sea, sometimes in a calm and windless situation, you will suddenly see strange phenomena such as ships, islands, towers and city walls reflected in the air. When the wind is strong, this scene will disappear immediately. This is an illusion in the atmosphere, called a mirage. This phenomenon occurs not only on the sea surface, but also in the air layer of lakes, rivers, deserts and Gobi. China Song Dynasty Shen Kuo recorded this illusion in Meng Qian Bi Tan: "There are clouds in Dengzhou Sea, such as palace terraces, city figures, chariots and horses, which are vivid in my mind." Dengzhou, now Penglai County, Shandong Province, is located on the south bank of the Bohai Sea, with Miaodao Islands in the Bohai Sea, dozens of kilometers away from Penglai. What is recorded here is the illusion of miaodao islands seen by Penglai.

In ancient times, because it was impossible to explain the causes of this phenomenon scientifically, it was endowed with various myths and legends, and a natural mirage was regarded as a mirage. In fact, the mirage is formed by the strong abnormal refraction of light in the atmosphere under special meteorological conditions. The refraction of light refers to the phenomenon that the speed of light changes and the direction of progress twists and turns under different air densities. When you insert a straight rod obliquely into the water, it looks like it is broken from the side! This is caused by light refraction. In summer, the temperature of seawater is low during the day, and the lower air is colder than the upper air due to the influence of water temperature, resulting in the inversion phenomenon that the upper air is cold and the lower air is warm, so the lower air density is particularly high, while the upper air density is particularly low. When the light of the distant scene gradually refracts from the dense air layer to the sparse air layer, it is totally reflected in the upper layer and then returns to the dense air in the lower layer. After this winding route, a mirage will appear in the eyes of the observer. Because people's vision always feels that the image comes from a straight line, the image we see is much higher than the real thing. This phenomenon is also called a mirage.

In the desert, because the sand is scorched by the sun during the day, the air near the sand layer rises very fast, forming a temperature distribution in which the lower layer is hot and the upper layer is cold, resulting in the phenomenon that the air density in the lower layer is much lower than that in the upper layer. At this time, the light of the front scene will be refracted from the dense air to the less dense air, thus forming a mirage. From a distance, it looks like a reflection in the water. People who travel long distances in the desert are hot and thirsty. When they see a mirage, they often mistakenly think that they have reached the cool lake. However, a gust of wind and sand swept through, and it is still an endless desert. This scene turned out to be just an illusion.

There is a kind of "gravel circle" in the desert, which is wonderful. It is a big stone that expands with heat and contracts with cold for hundreds of years, and it is rolled again and again, forming a round gravel circle on the ground, much like a manual arrangement, but it is actually formed naturally.