Ice falling in the air-What are the causes and hazards of hail?

Hail is also called "hail", commonly known as hail, and in some areas it is called "Lengzi", which is the most common in summer or at the turn of spring and summer. It is some ice particles as small as mung beans and soybeans and as big as chestnuts and eggs. Except Guangdong, Hunan, Hubei, Fujian, Jiangxi and other provinces, China suffers from hail disasters in different degrees every year, especially in mountainous and hilly areas in the north. The terrain is complex, the weather is changeable, and there are many hail disasters, which are very harmful to agriculture. Heavy hail destroyed crops and houses, injured people and killed livestock. Extraordinary hail can even be bigger than grapefruit, which will kill people, destroy large areas of farmland and trees, and destroy buildings and vehicles. And it has a strong lethality.

Ice Drops in the Air-Hail

Hail is a kind of solid precipitation. It is a spherical or conical ice block, which consists of transparent layers and opaque layers alternately. The diameter is generally 5 ~ 50 mm, and the maximum can reach 10 cm. The greater the diameter of hail, the greater the destructive power. Hail often destroys crops and threatens the safety of people and animals. This is a serious natural disaster. Hail comes from strong convective convective clouds (cumulonimbus clouds). The updraft in the cloud is stronger than that in the general thunderstorm cloud. Small hailstones are formed by hail embryos colliding with supercooled water droplets several times up and down in convective clouds, and fall to the ground when the updraft in the clouds can't support them. Heavy hail is produced in a thunderstorm cloud with strong updraft and rich liquid water. The scope of each hail is very small, usually several meters to several kilometers wide and 20-30 kilometers long, so there is a folk saying that "hail hits a line". Hail mainly occurs in mid-latitude continental areas, usually more in mountainous areas than in plains, and more in inland areas than in coastal areas. Hail in China mostly occurs in spring, summer and autumn, accounting for about 70% of the total from April to July. Severe hail disaster areas include southern Gansu, Longdong, the yinshan mountains, Taihang Mountain, western Sichuan and Yunnan Province.

Therefore, it is necessary for us to understand the temporal and spatial turbulence pattern of hail disaster and the losses caused by hail disaster, so as to better prevent hail disaster and reduce economic losses.

Formation of hail

Hail falls from the clouds like rain and snow. But the clouds that hail are very strong cumulonimbus clouds, and only very strong cumulonimbus clouds can hail. Cumulonimbus clouds, like all kinds of clouds, are formed by the rising and condensation of air near the ground. Air rises from the ground, and in the process of rising, the air pressure decreases and the volume expands. If there is no heat exchange between the rising air and its surroundings, the air temperature will decrease due to the energy consumed by expansion. This temperature change is called adiabatic cooling. According to the calculation, every time the air in the atmosphere rises 100 m, the temperature will decrease by about 1℃ due to adiabatic change. We know that at a certain temperature, there is a limit to the amount of water vapor contained in the air, and reaching this limit is called "saturation". When the temperature is lowered, the amount of water vapor that may be contained in the air will decrease. Therefore, the originally unsaturated air may reach saturation due to adiabatic cooling in the upward movement. After the air reaches saturation, excess water vapor will attach to the floating condensation nuclei in the air and form water droplets. When the temperature is lower than 0℃, excess water vapor will condense into fine ice crystals. These water droplets and ice crystals gather together and float in the air to become clouds. There are different forms of air movement in the atmosphere, forming different forms of clouds. Clouds formed by convection include shallow cumulus, thick cumulus and cumulonimbus, which are collectively called cumulonimbus. They are isolated and upward-developing clouds, because there are upward and downward movements in convection, which often form clouds in the updraft area, become gaps in the downdraft area, and sometimes you can see the blue sky.

Cumulus clouds form various clouds due to different convective intensities, and the sizes of clouds vary greatly. If the convective movement in the cloud is weak and the updraft can't reach the condensation height, there will be no cloud, only dry convection. If the convection is strong, it can develop into cumulus clouds. The top of cumulus clouds is like broccoli, which is composed of many well-defined convex clouds, and the cloud thickness can reach 4~5 kilometers. If the convection is intense, a cumulonimbus cloud can be formed, the cloud bottom is dark, the cloud top develops very high, reaching about 10 km, the edge of the cloud top becomes blurred, and the cloud top often expands to form an anvil. Generally, cumulonimbus clouds may produce thunderstorms, but only when they are particularly strong, the clouds are high, there is strong rising gas in the clouds, and there is a lot of water in the clouds will hail be produced. This kind of cloud is usually called hail cloud.

Hail clouds consist of water droplets, ice crystals and snowflakes. Generally, there are three layers: ① The bottom layer is above 0℃ and consists of water droplets; ② The temperature of the middle layer is 0~-20℃, which is composed of supercooled water droplets, ice crystals and snowflakes; ③ The temperature of the top layer is below -20℃, which is basically composed of ice crystals and snowflakes.

In the hail cloud, the airflow is very strong. Usually in the direction of the cloud, a strong updraft enters from the bottom of the cloud and flows out from the top of the cloud. There is also a downward airflow flowing in from the middle behind the cloud and out from the bottom of the cloud. This is also the precipitation area where hail usually occurs. These two organized updrafts are connected with the airflow between the downdraft and the environment, so the airflow structure in the heavy hail cloud is relatively continuous. The strong updraft not only delivers enough water vapor to the hail cloud, but also supports the hail particles to stay in the cloud and grow to a considerable size before landing.

Hail falls from the clouds like rain and snow. It is a kind of solid precipitation falling from cumulonimbus clouds.

The formation of hail requires the following conditions:

(1) There must be a fairly thick unstable layer in the atmosphere.

(2) Cumulonimbus clouds must develop to a height that can freeze a single large water droplet (generally, the temperature is-12 ~- 16℃).

(3) There should be strong wind shear.

(4) The vertical thickness of the cloud should not be less than 6 ~ 8km.

(5) Cumulonimbus clouds are rich in water. Generally, it is 3 ~ 8g/m3, and there is an accumulation area of liquid supercooled water above the maximum rising speed.

(6) There should be an inclined, strong and uneven updraft in the cloud, which is generally above10 ~ 20m/s. ..

How does hail grow in hail clouds? In the hail cloud, the strong updraft carries many large and small water droplets and ice crystals, some of which fuse with ice crystals and freeze into larger ice particles. These particles and supercooled water droplets are transported by the updraft to the water content accumulation area, which can become the hail core. The initial growth core of these hailstones has good growth conditions in the water content accumulation area. After the hail enters the growth area with the updraft, it collides with supercooled water droplets in the area with large water volume and low temperature, and grows into a transparent ice layer, and then enters the low temperature area with small water volume, where it is mainly composed of ice crystals, snowflakes and a small amount of supercooled water droplets, and the hail adheres to them and freezes to form an opaque ice layer. At this time, the hail has grown up and the updraft there is weak. When it can't support the growing hail, the hail will fall in the updraft and continue to grow through the fusion of ice crystals, snowflakes and water droplets in autumn. When it falls to a higher temperature area, the supercooled water droplets that hit it will form a transparent ice layer. At this time, if it falls into another stronger updraft area, the hail will rise again and repeat the above growth process. In this way, the hail grows transparently; Due to the differences in growth time and water content, the thickness and other characteristics of each layer are also different. Finally, when the updraft couldn't support the hail, it fell from the clouds and became the hail we saw.

Characteristics of hail

Generally speaking, hail has the following characteristics:

(1) has strong locality, and the influence range of each hail is generally about tens of meters to several kilometers wide and hundreds of meters to more than ten kilometers long; (2) The duration is short, a rainstorm or hail usually lasts only 2 ~ 10 minutes, and a few last more than 30 minutes; (3) It is significantly affected by the terrain, and the more complex the terrain, the greater the possibility of hail; (4) The interannual change is great. In the same area, some years occur several times in a row, and some years occur several times or even do not occur; (5) The occurrence area is wide, and it can occur in a vast climatic zone from subtropical zone to temperate zone, but the frequency of occurrence is mostly in temperate zone.

Classification of Hail According to the diameter of the largest hail (general hail), the cumulative time of hail falling and the thickness of hail accumulation, hail is divided into three grades.

(1) light hail: most hail is less than 0.5cm in diameter, the cumulative hail suppression time is less than 10 minute, and the hail thickness in the ground area is less than 2cm(2) medium hail: most hail is 0.5-2.0 cm in diameter, the cumulative hail suppression time is10-30 minutes, and the hail thickness is 2. (3) Heavy hail: Most hailstones have a diameter of more than 2.0 cm, the cumulative hail time is more than 30 minutes, and the hail thickness is more than 5 cm.

Harm of hail

Hail disaster is a serious meteorological disaster caused by strong convective weather system. Although it appears in a small range and in a short time, it is fierce and strong, and is often accompanied by sudden disastrous weather processes such as strong winds, heavy precipitation and sharp cooling. China is a country with frequent hail disasters. Hail brings huge losses to agriculture, construction, communication, electricity, transportation and people's lives and property every year. According to relevant statistics, the economic losses caused by hail in China reach hundreds of millions or even billions of yuan every year.

Hail damages crops.

Many people have encountered hail in thunderstorm weather. Usually, these hailstones will not exceed the size of softball, they fall from the storm clouds. However, sometimes the hail is very large. There used to be 128 kg of hail falling from the sky, which would break into many small pieces when it fell to the ground. The most mysterious thing is that when the sky is cloudless, huge hail falls from the sky. There have been many incidents that confirmed that the wings of airplanes were attacked by hail. At present, scientists can't explain why there is such a huge hail.

Hail prevention and control (1) forecast. Since 1980s, with the wide application of advanced equipment such as weather radar, satellite cloud image receiving, computer and communication transmission in meteorological services, the ability to track and monitor hail activities has been greatly improved. When the local meteorological observatory (station) finds hail weather, it shall immediately notify the meteorological observatory (station) that may be affected. Meteorological departments at all levels combine modern meteorological science and technology with long-term accumulated forecasting experience to comprehensively forecast the occurrence, development, intensity, scope and harm of hail, and continuously improve the forecasting accuracy. In order to transmit the hail warning information to government leaders and the masses at all levels as soon as possible, meteorological departments at all levels issued "alerts" and "emergency alerts" through local radio stations, television stations, telephones, computer service terminals and severe weather warning systems, which enabled all sectors of society and the broad masses of the people to take defensive measures in advance, avoided and alleviated disaster losses, and achieved obvious social and economic benefits.

(2) Prevention and control. China is one of the countries with early artificial hail suppression. Due to the serious hail disaster in China, the hail suppression work has received the attention and support of the government. At present, many provinces have established long-term experimental fields, conducted rigorous experiments and achieved many valuable scientific research results. Artificial hail suppression is carried out to make it develop in the direction people expect and achieve the purpose of disaster reduction. At present, the commonly used methods are as follows: ① Use rockets, anti-aircraft guns or airplanes to send catalysts such as silver iodide, lead iodide and dry ice directly into the cloud. (2) On the ground, catalysts such as silver iodide, lead iodide and dry ice are sent to the free atmosphere before the formation of cumulonimbus clouds, so that these substances can play the role of hail embryos in hail clouds, increase hail embryos and reduce hail. (3) Fire rockets and anti-aircraft guns at hail clouds on the ground, or fire rockets and bombs at hail clouds on airplanes, thus disrupting the water transport to hail clouds. (4) Sprinkle condensation nuclei on warm clouds with rockets and anti-aircraft guns to make the clouds form precipitation, so as to reduce the water in the clouds; In the cold cloud, some ice cores are scattered to inhibit the growth of hail embryos.

(3) Agricultural hail suppression measures. The common methods are as follows: ① Planting grass and trees in hail-prone areas, increasing forest area, improving landform environment and destroying hail cloud conditions, so as to reduce hail disasters; (2) Planting more crops with strong hail resistance and resilience; ③ timely harvest mature crops; (4) In the hail season in hail-stricken areas, farmers carry hail suppression tools with them in the fields, such as bamboo baskets and wicker baskets, to reduce personal injuries.