Low visibility disaster

Fog is produced by the condensation of water vapor in the atmosphere into small water droplets (2 ~ 15? M) Water vapor phenomenon, which is concentrated in the gas layer near the ground, reduces the horizontal visibility to less than 1000 meters.

The places with the most foggy days in China are Emei Mountain Meteorological Station, with an average foggy day of 322d, foggy city of Chongqing of 69d (the longest year 148d), Mengla 153d in Xishuangbanna, Yunnan, and Taining 154d in northwest Fujian.

There are many condensation nuclei in the urban air, and when the humidity is high, there is more fog in the city than in the suburbs. However, the urban heat island effect is obvious. When the humidity has just reached saturation, the fog in the suburbs is inversely proportional to the city.

Heavy fog often leads to traffic accidents.

(2) Sandstorm

Sandstorm is an atmospheric turbidity phenomenon in which strong winds blow up a lot of dust on the ground and make the horizontal visibility less than 1km.

1. Classification

Sandstorm weather can be divided into four categories: floating dust, sand blowing, sandstorm and strong sandstorm.

Floating dust: a weather phenomenon in which dust and fine sand float evenly in the air, making the horizontal visibility less than 10km;

Sand blowing: the weather phenomenon that the wind blows up the dust on the ground, making the air quite turbid and the horizontal visibility is within 1 ~ 10 km;

Sandstorm: a weather phenomenon in which strong winds blow up a lot of dust on the ground, making the air very turbid and the horizontal visibility is less than 1km;

Strong sandstorm: a weather phenomenon in which strong winds blow up dust on the ground, making the air turbid and the horizontal visibility less than 500m m.

Step 2 distribute

Sandstorms are mainly distributed in arid and semi-arid areas, and the necessary conditions are as follows: ① loose and dry ground soil with little vegetation; (2) The wind is strong enough and lasts for a long time; (3) Atmospheric stratification is very unstable. Drought and lack of rain in the early stage, warming weather and rising temperature are the special weather background for the formation of sandstorms; The convective cells before the ground cold front develop into clouds or squall lines, which is a small and medium-sized system conducive to the development and strengthening of sandstorms. The topographic condition conducive to the increase of wind speed, that is, the narrow tube effect, is one of the favorable conditions for the formation of sandstorms.

According to the data of Beijing Meteorological Observatory, during the period of 195 1 ~ 2000, the average annual low visibility disasters were: 22 days of strong wind, 20 days of sand blowing, 6 days of floating dust and 2.4d days of sandstorm. The most frequent year of sandstorm is 1 20 days in 966, with an average of 5 days in 1950s and 1960s and less than1day in 1970s. At the beginning of 2002, the research group of "Study on the Influence of Sandstorm and Yellow Sand on Atmospheric Particulate Matter in Beijing" of the State Environmental Protection Bureau announced that the source area and transmission path of sandstorm in Beijing had been preliminarily identified. Most sandstorms in Beijing come from overseas, and there are three transmission paths for sandstorms to enter Beijing: North Road, Northwest Road and West Road. The path of sandstorm weather in China can be divided into northwest path, west path and north path: northwest path 1. Sandstorm weather generally originates in the central and western plateau of Mongolia or Alashan Plateau in western Inner Mongolia, and mainly affects northwest and north China. Northwest Route 2. Dust weather originated in southern Mongolia or central and western Inner Mongolia, mainly affecting the eastern part of northwest China, northern North China and most of northeast China; To the west, dusty weather originated in Gobi area in southwest or south Mongolia and desert area in western Inner Mongolia, mainly affecting northwest and north China. On the northern route, dusty weather generally originates in the vast area south of Ulaanbaatar, Mongolia, and mainly affects the eastern part of northwest China, most of north China and southeast China. From March 8, 2002 to March 2, 2002, the sandstorm weather process with the largest scope, strongest intensity, most serious impact and longest duration in recent 20 years hit the land of more than 400,000 square kilometers in northern China, affecting the population of1300,000.

3. Hazards of sandstorm weather

Sandstorm weather is a disastrous weather in northwest China and northern North China, which can cause houses to collapse, traffic power supply to be blocked or interrupted, fires, casualties, etc., pollute the natural environment, destroy crop growth, and cause serious losses and great harm to national economic construction and people's lives and property safety (Figure 8-2).

Figure 8-2 On May 9, 2006, a sandstorm hit Khartoum, the capital of Sudan.

(according to)

4 sandstorm weather forecast and early warning release standard

(1) decision service

When there is expected to be a sandstorm weather process in the next 24 hours, the sandstorm weather forecast will be released in internal bulletins, special reports and decision-making service materials.

(2) Public forecast

National standard: It is expected that there will be a sandstorm weather process in the next 24 hours, and when it has a great impact or affects Beijing and Tianjin, it will issue a sandstorm warning to the public. When it is predicted that there will be a sandstorm or strong sandstorm weather process in the next 24 hours and it will cause serious impact, it will issue a sandstorm warning to the public.

Provincial standards: determined by the meteorological bureaus of all provinces (autonomous regions and municipalities) with reference to national standards.

Think about a problem

1. Analyze the causes and effects of the low temperature and freezing disaster in southern China in 2008.

2. How are the main sand sources of sandstorms affecting northern China formed?