Why do sandstorms form?

Sandstorm weather is the general name of sandstorm, sand blowing and floating dust weather, and it is a phenomenon of atmospheric turbidity caused by strong wind blowing up ground dust or being carried to downstream areas by high-altitude airflow. According to the international observation standards, the meteorological department of our country divides the dusty weather with visibility less than 1 km into sandstorm, sand blowing and floating dust. Sandstorm weather with horizontal visibility less than 1 km is called sandstorm; The horizontal visibility within 500 meters is called strong sandstorm. In some areas of northern China, those with horizontal visibility less than 50 meters are called extremely strong sandstorms, and some are also called black winds. So we say that sandstorm is a strong sandstorm weather phenomenon. It is a frequent disastrous weather in arid and semi-arid areas.

2. Hazards of dusty weather

In China, the frequency of extremely strong sandstorms is not much, but sandstorms with average intensity and strong sandstorms will appear in northern China every spring. This kind of weather has a wide influence. It has seriously polluted the atmospheric environment and affected people's health and life. More seriously, intuitively speaking, it scraped away the topsoil of the upstream farmland, reduced the soil quality, exposed the roots of crops and pastures, and led to the reduction of agricultural production in that year; Most importantly, this kind of wind erosion has caused extensive damage, causing large areas of farmland, grassland, grassland and woodland to be desertified and buried, which has aggravated desertification in China. Especially in some areas with serious desertification in China, where nothing grows, local herders have to move and find new places to live.

3. The cause of dust weather.

First of all, we must understand the necessary conditions for the formation of sandstorms. These are dust sources, strong winds and unstable atmospheric stratification. It is easy to understand that strong wind is the driving force of sandstorm and the source of sandstorm is its material basis. In spring, cold air and cyclone activities are frequent, and the accompanying strong wind is the starting factor of sandstorm; Drought and lack of rain in spring is a major climate feature in northern China, and the continuous dry and warm climate has caused soil looseness, which has the material basis for sandstorms. Under the condition of partly cloudy clear sky and strong radiation from the sun, the near-surface gas layer is heated around noon, which is in a state of thermal instability, which is very beneficial to the development of air convection and the exchange of momentum and energy, and makes the dust particles in the atmosphere rise higher and spread faster. If the cold and warm air masses cross the front, the strong uplift near the front area can exchange energy and momentum, and the thermal uplift of the gas layer is easy to form strong sandstorm weather. Therefore, in spring, especially in the afternoon to evening in spring, sandstorms often form.

4. The relationship between the intensity and frequency of sandstorm weather and climate change.

According to our statistics, in most years of the 1990s, the cold air activity in spring in China was relatively weak, with relatively more precipitation and less dusty weather. However, after the light rain and drought in 1999 and 2000, there was continuous drought in winter and spring in 200 1 year, and the spring temperature in northern China was obviously higher than normal. In some areas, dry and hot weather above 35 degrees even appeared, which accelerated the evaporation and dissipation of soil moisture and aggravated the drought, resulting in poor vegetation growth and recovery, looser soil and easy dust falling. In the past two years, there have been frequent cyclones and cold air activities, so there are more dusty weather. It can be seen that in the climate background of little rain and drought, it is often the high incidence period of dust weather.

As mentioned above, there are three necessary conditions for the formation of sandstorms. Studies have shown that the desertification rate in China is accelerating, that is to say, the overall land cover in China is deteriorating, and the material sources of sandstorms are getting richer and richer, so the number of sandstorms should increase. However, according to the statistics of 1954-200 1 nearly 48 years, the number of sandstorms in northern China is obviously less than that in 1950s and 1960s. That is to say, further statistics show that the average afternoon wind speed of 6 m/s (i.e. the critical wind speed for dust) measured by most meteorological production in northern China is also smaller in 1990s than that in 1950s and 1960s, which is consistent.

As mentioned above, the causes of sandstorm weather have obvious stages in the time evolution of sandstorm phenomenon. Frequent periods and low frequency periods appear alternately. There are about five frequent periods in the last thousand years, and each frequent period is about 90 years. The frequent occurrence of sandstorms is related to the climate background, and also corresponds to the climate background of large-scale drought. In addition, this is related to many cold waves and strong winds in cold climate. Recent studies have also pointed out that in cold weather, the influence of sandstorms is wider than that in warm weather, and its southern boundary can reach 23 north latitude. 5 degrees. The difference in frequency and influence range of sandstorms under different climatic backgrounds can be attributed to the obvious difference in weather circulation situation.