What is the cause of sandstorm?

1. The giant sandstorm originated from soil wind erosion.

China Academy of Sciences once carried out an experiment to explore sand and dust. Some experts and engineers in cold and dry areas found that soil wind erosion is the primary link in the formation of sandstorms through simulation experiments in sandstorm tunnels. Wind is the main driving force of soil erosion, among which the nature of airflow, wind speed and related conditions of wind action in soil wind erosion are the most important factors. In addition, the moisture content in soil, that is, the degree of wetness, is also one of the important reasons that affect soil wind erosion.

According to the experimental results, it is proved that rich vegetation is one of the effective methods to prevent sandstorms. Plants usually affect wind erosion in three forms: a certain wind momentum scattered on the ground; Reduce the spread between airflow and dust; Stop the movement of soil, dust, etc. After experimental research, experts also reached an important conclusion: Sandstorm is not only the product of certain natural conditions, but also closely related to human activities. For example, excessive deforestation and overgrazing will destroy plants' natural defense against sandstorms. The construction of industrial and mining traffic is also an important reason for the over-exploitation of wasteland and the destruction of vegetation. Many human activities will disturb the natural structure of the ground and form a large area of desertified land, which is the direct cause of the formation and bad development of sandstorms.

2. The culprit of sandstorm: atmospheric circulation.

The brief scene of spring sandstorm in Beijing every year is just a weather process that the 300,000-square-kilometer Loess Plateau in northern China has experienced every year for two or three million years. The difference is that the wind of the latter is stronger, and the wind lasts longer, even for several days. The source of dust is the desert and Gobi hundreds of kilometers away, not the crossroads that people simply think is 50 meters away. The formation of sandstorms is like God grabbing a lot of sand on the surface of Gobi Desert in northwest China and Central Asia and throwing it to the southeast. This phenomenon existed as early as 2.4 million years ago. After the Indian plate moved northward and collided with the Eurasian plate, the Indian continental crust was inserted under the Asian continental crust to prop up the latter. As a result, the shallow sea of the Himalayas disappeared, the Himalayas began to form and gradually lifted, and the Qinghai-Tibet Plateau was also squeezed and lifted by the Indian plate. This activity lasted for more than 60 million years. By about 2.4 million years ago, the altitude of the Qinghai-Tibet Plateau should have reached more than 2,000 meters.

This huge change in surface morphology directly changed the pattern of atmospheric circulation. Long ago, the Pacific Ocean was to the east of Chinese mainland. Today, Siberia in the north and Himalaya in the south were occupied by shallow seas at that time, and the Mediterranean in the west also extended to far places in Central Asia at that time. Therefore, most of the flat Chinese mainland can be nourished by warm and humid ocean air, and the climate is warm and humid. The northwest of China and the interior of Central Asia are dominated by subtropics, and there are no large-scale deserts and Gobi in some places. However, the east-west Himalayan mountains stopped the warm and humid air mass in the Indian Ocean from moving northward. With the passage of time, the northwest of China has become more and more arid, gradually forming a large area of desert and Gobi. This is the birthplace of dust accumulated on the Loess Plateau. The huge Qinghai-Tibet Plateau just stands in the westerly belt of the northern hemisphere, and its height has been increasing for 2.4 million years. The width of the Qinghai-Tibet Plateau accounts for about one third of the westerly belt, which divides the surface of the westerly belt into two branches. The south branch flows eastward along the south side of the Himalayas, and the north branch flows eastward from the northeast edge of the Qinghai-Tibet Plateau. This kind of high-altitude airflow exists at an altitude of 3500 ~ 7000 meters all the year round and becomes the main driving force to carry dust. At the same time, the Qinghai-Tibet Plateau is too high, which strengthens the East Asian monsoon climate. The east wind blowing from northwest to southeast meets the west wind, so the Loess Plateau appeared in northern China.

The temperature in the northwest of China and the inland of Central Asia changes dramatically, so the rocks in these places disintegrate faster than those in other places and eventually become fragments. Geologists divide these debris into: gravel, more than 2 mm; Sand, 2 ~ 0.05 mm; Silt, 0.05 ~ 0.005 mm, clay, less than 0.005 mm, in which clay and silt particles can be brought to a height of more than 3500 meters. After entering the westerlies, the wind-blown debris will drift to the southeast with westerly jet and gradually fall to the ground until it reaches the middle and lower reaches of the Yellow River.

For millions of years, the process of transporting sand from northwest to southeast in Asia has never stopped. A lot of sand falls in the area where the Loess Plateau is located, and even many hills in North China, such as Wutai Mountain and Taihang Mountain, have loess accumulation. Of course, the scouring effect of several large rivers and countless valleys in northern China, including the Yellow River, is just the opposite of the accumulation of loess. Otherwise, the loess plateau will not be like this, and the thickness will not exceed 409.93 meters. The North China Plain, east of Taihang Mountain, is also a sand subsidence area, but it is a subsidence area, and many rivers have developed at the same time, so most of the falling sand will be washed away by rivers or buried by the sediment brought by rivers.

There are hundreds of records about rain soil, rain loess, rain yellow sand and rain haze in China ancient books. The earliest record of "rainy soil" can be traced back to 1 150 BC: "The sky is yellow and foggy, and sand falls like rain from the sky." This is actually a record of a sandstorm. The location of rainy soil is mainly in the Loess Plateau and its vicinity. The ancients regarded this kind of thing as a strange catastrophe phenomenon and thought it was a sign of "harmony between man and nature" It is recorded in the Natural History compiled by Zhang Hua in the Jin Dynasty: "When Xia Jie was in the deep valley, the night palace was mixed with men and women, and he did not go out to listen to politics for ten days. It was a windy day and filled the empty valley overnight. "