What is a sandstorm?
As a kind of high-intensity sandstorm disaster, sandstorm can't happen in all windy places, but only in those areas with dry climate and sparse vegetation. In the northwest of China, the forest coverage rate is not high, and the poor northwest people want to make a fortune by digging licorice, holding seaweed and mining. These predatory acts of destruction further aggravated the sandstorm disaster in this area. Exposed land is easily rolled up by strong winds, forming sandstorms or even strong sandstorms.
There are many hazards of sandstorm: 1, people and animals die, buildings collapse and agricultural production is reduced. Sandstorms are as harmful to people, animals and buildings as typhoons and tornadoes. In the past five years, the northwest of China has been hit by sandstorms for more than 20 times, resulting in economic losses of more than 654.38+0.2 billion yuan, and more than 200 people have died and disappeared.
2. Air pollution and topsoil loss. Dust storms contain at least 38 chemical elements, which greatly increase the concentration of atmospheric solid pollutants and cause long-term and potential harm to the atmospheric environment, soil and agricultural production in the origin, surrounding areas and downwind areas. Especially after the meager topsoil on which crops depend is scraped away, the barren land will seriously affect the yield of crops.
Sandstorm has three elements: strong wind, sand source and unstable air.
strong breeze
Persistent strong winds are the dynamic conditions for the formation of sandstorms. For example, it is observed that when a strong sandstorm forms, if the wind speed reaches 30m per second (11wind), coarse sand (0.5- 1.0mm in diameter) will fly out of the ground for several tens of centimeters, and fine sand (0. 125-0.25mm in diameter) will fly out.
Shayuan
China is one of the countries with the most deserts in the world. Northwest China, North China and Northeast China are places where deserts and sandstorms are concentrated. The desert and sandy land here covers an area of over 700,000 square kilometers. Various sand dunes in the desert can be divided into mobile sand dunes, semi-fixed sand dunes and fixed sand dunes according to their stability. When sandstorms occur, the amount of dust raised by moving dunes is the largest, the semi-fixed dunes are smaller, and the fixed dunes are the smallest.
Except desert and sandy land, most of northern China belongs to mid-latitude arid and semi-arid areas, and the ground is mostly sparse grassland and arid cultivated land, with sparse vegetation and serious man-made damage. In spring, the ground warms up and thaws, the ground is bare, the wind blows wildly, and the dust is diffuse, forming dusty weather in some areas and areas where the wind passes.
Unstable air
Let's look at an example in life first. When you fiddle with the stove, the fire burns brightly. With a slight poke, ash will often fly all over the house. When the fire goes out, you usually don't spend much effort to raise dust, which involves the problem of air stability. After the fire is extinguished, there is little difference in air temperature between the top and bottom of the stove, so the air is stable. When the fire burns violently, the air near the fire is hot, and the air far away from the fire is cold. Hot air is lighter than cold air and tends to rise, so the air above the fire is unstable. In this way, the stirred furnace ash can easily rise with the hot air and then float all over the room.
In nature, so do sandstorms. If the lower air temperature is low and stable, the dust blown by the wind will not be lifted very high; If the lower air temperature is high, it will be unstable and easy to move upward. After the wind blows, the dust will be blown high, forming a sandstorm. In fact, sandstorms in China are usually strongest in the afternoon or from afternoon to evening, because this is the most unstable time of the day.
In addition to the above three factors, human production activities and other factors are also very important to the formation of sandstorms. Such as man-made destruction of vegetation, industrial and mining traffic construction, large-scale construction and other damage to the surface, provide fine dust for the occurrence and development of sandstorms.
In meteorology, sandstorm weather is usually divided into three grades, namely floating dust, sand blowing and sandstorm.
Floating dust refers to dust and fine sand that float evenly in the air and make the horizontal visibility less than 10 km. Most of the floating dust comes from distant dust spread by high-altitude airflow, or from fine particles floating in the air after sandstorm and sand blowing.
Sand blowing refers to the strong wind blowing away the dust on the ground, making the air quite turbid, and the horizontal visibility is in the range of 1 km to1km.
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Sandstorm refers to the strong wind blowing a lot of dust on the ground, which makes the air very turbid and the horizontal visibility is less than 1 km.
Strong sandstorm (the instantaneous wind speed is more than 25m per second and the wind force is more than 10) may make the horizontal visibility on the ground less than 50m, which is extremely destructive, commonly known as "black wind".