Why does hail fall? Where does hail come from?

Hail Catalog [Hide]

[Edit this paragraph] Overview

Hail (Hail)

Hail, also called "hail" , commonly known as hailstones, or "lengzi" in some areas, are most common in summer or at the turn of spring and summer. They are ice particles as small as mung beans and soybeans, and as large as chestnuts and eggs. Extra large hailstones are larger than grapefruits. Except for Guangdong, Hunan, Hubei, Fujian, Jiangxi and other provinces where hail is relatively rare, all regions in my country are subject to varying degrees of hail disasters every year. Especially in the mountainous and hilly areas of the north, the terrain is complex, the weather is changeable, hail is abundant, and damage is severe, which is very harmful to agriculture. The violent hail destroys crops, damages houses, people are injured, and livestock are killed. Happens often. Therefore, hail disaster is one of the serious disasters in my country.

A solid precipitation substance. It is a spherical or conical ice cube composed of alternating transparent and opaque layers. The diameter is generally 5 to 50 mm, and large ones can sometimes reach more than 10 cm. They are also called hail or hailstones. Hail often damages crops and threatens the safety of people and animals. It is a serious natural disaster. Many countries with severe hail disasters have carried out artificial hail prevention experiments.

The larger the hailstones, the more destructive they are. Hail falls from cumulonimbus clouds with particularly strong convection, and the updrafts in the clouds are stronger than those in ordinary thunderstorm clouds. Small hailstones grow in convective clouds from hail embryos colliding with supercooled water droplets up and down several times. When the updraft in the cloud cannot support them, they fall to the ground. Large hailstones are produced in thunderstorm clouds with a strong upward airflow and abundant liquid water content. The scope of each hailstorm is very small, generally ranging from a few meters to several kilometers in width and 20 to 30 kilometers in length, so there is a folk saying that "hail strikes a line." Hail mainly occurs in mid-latitude continental areas, usually more in mountainous areas than in plains, and more inland than along the coast. Hail in China mostly occurs in spring, summer and autumn, with April to July accounting for about 70% of the total occurrences. The more severe hail disaster areas include southern Gansu, Longdong, the Yinshan Mountains, the Taihang Mountains and the western areas of Sichuan and Yunnan provinces.

Hail disaster is a severe meteorological disaster caused by strong convective weather systems. Although its range is small and its duration is relatively short, it is fierce and intense, and is often accompanied by strong winds and Paroxysmal disastrous weather processes such as heavy rainfall and rapid cooling. China is a country where hail disasters occur frequently. Hail causes huge losses to agriculture, construction, communications, electricity, transportation, and people's lives and properties every year (Figure 1, Figure 2). According to relevant statistics, the economic losses caused by hail in my country every year amount to hundreds of millions or even billions of yuan. Therefore, it is necessary for us to understand the spatial and temporal turbulence pattern of hail disasters and the losses caused by hail disasters, so as to better prevent hail disasters and reduce economic losses.

[Edit this paragraph] Hail formation

Hail, like rain and snow, falls from the clouds. However, the hail cloud is a very powerful cumulonimbus cloud, and only a particularly powerful cumulonimbus cloud can produce hail.

Cumulonimbus clouds, like all kinds of clouds, are formed by the rising and condensation of air near the ground. When the air rises from the ground, the air pressure decreases and the volume expands during the rise. If there is no heat exchange between the rising air and the surroundings, the air temperature will decrease due to the energy consumed by the expansion. This temperature change is called adiabatic cooling. According to calculations, for every 100 meters the air rises in the atmosphere, the temperature will drop by about 1 degree due to adiabatic change. We know that at a certain temperature, there is a limit to the amount of water vapor the air can hold. Reaching this limit is called "saturation". When the temperature decreases, the amount of water vapor that the air can hold will decrease. Therefore, the originally unsaturated air may reach saturation due to adiabatic cooling during its upward movement. After the air reaches saturation, the excess water vapor will adhere to the condensation nuclei floating in the air and form water droplets. When the temperature drops below zero degrees Celsius, excess water vapor condenses into tiny ice crystals. These water droplets and ice crystals gather together and float in the air to form clouds.

There are various forms of air movement in the atmosphere, forming clouds of different shapes. Clouds formed due to convective motion include Cumulus leucocumulus, Cumulus congestus, and Cumulonimbus. They are collectively called cumulus clouds. They are all isolated and upward-developing clouds. Because there are ascending and sinking movements in convection, clouds are often formed in the updraft area, while they become gaps in the clouds in the downdraft area. Sometimes the blue sky can be seen. .

Cumulus clouds form various cloud shapes due to different convection strengths, and their cloud sizes vary greatly. If the convective movement within the cloud is very weak and the updraft does not reach the condensation height, no cloud will be formed, only dry convection. If the convection is strong, it can develop into Cumulus Congestus. The top of Cumulus Congestus is like a cabbage, composed of many convex cloud bubbles with clear outlines. The cloud thickness can reach 4-5 kilometers. If the convection movement is very violent, cumulonimbus clouds can be formed. The cloud base is dark and dark, the cloud top develops very high, up to about 10 kilometers, the edge of the cloud top becomes blurred, and the cloud top often expands to form an anvil shape. Generally, cumulonimbus clouds may produce thunderstorms, but only very strong cumulonimbus clouds with very tall clouds, strong rising gases in the clouds, and abundant moisture in the clouds can produce hail. This type of cloud is also usually called Hail cloud.

Hail clouds are composed of water droplets, ice crystals and snowflakes.

Generally there are three layers: the bottom layer has a temperature above 0°C and is composed of water droplets; the middle temperature is 0°C to -20°C and is composed of supercooled water droplets, ice crystals and snowflakes; the top layer has a temperature below -20°C. Basically made up of ice crystals and snowflakes.

The airflow in a hail cloud is very strong. Usually in the direction of the cloud, a very strong updraft enters from the bottom of the cloud and flows out from the upper part of the cloud. There is also a downdraft that flows in from the middle level behind the cloud and flows out from the bottom of the cloud. This is also the precipitation area where hail usually occurs. These two organized rising and falling airflows are connected with the ambient airflow, so the airflow structure in strong hail clouds is generally relatively continuous. The strong updraft not only transports sufficient water vapor to the hail cloud, but also supports the hail particles to stay in the cloud, making them grow to a considerable size before falling down.

Hail, like rain and snow, falls from clouds. It is a kind of solid precipitation that falls 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 individual large water droplets (the temperature is generally considered to be -12~-16℃).

③ There must be strong wind shear.

④The vertical thickness of the cloud cannot be less than 6 to 8 kilometers.

⑤Cumulonimbus clouds are rich in water. It is generally 3 to 8 g/m3. There is an accumulation zone of liquid supercooled water above the maximum rising speed.

⑥ There should be inclined, strong and uneven updrafts in the cloud, generally above 10 to 20 meters/second.

How do hailstones grow in hail clouds?

The strong updrafts in hail clouds carry many large and small water droplets and ice crystals in motion, including Some water droplets and ice crystals merge and freeze into larger ice particles. These particles and supercooled water droplets are transported to the moisture content accumulation area by the updraft, and they can become hail cores. These initial growth cores of hail have good growth in the moisture content accumulation area. condition. After the hail core is carried by the rising airflow into the growth zone, it collides with supercooled water droplets in an area with a large amount of water and a not too low temperature, and grows into a transparent layer of ice, and then moves upward into a low-temperature area with less water. Here it is mainly composed of It is composed of ice crystals, snowflakes and a small amount of supercooled water droplets. Hail nuclei stick to them and freeze to form an opaque ice layer. At this time, the hail has grown, and the updraft there is weak. When it cannot support the larger hail, the hail will fall in the updraft, and continue to merge with ice crystals, snowflakes and water droplets during the fall. When it falls to a higher temperature zone, the supercooled water droplets that collide with it form a transparent ice layer. If it falls into another stronger updraft area at this time, the hail will rise again and repeat the above-mentioned growth process. In this way, hailstones grow in layers of transparent and opaque layers; due to differences in growth time, moisture content and other conditions, the thickness and other characteristics of each layer are also different. Finally, when the updrafts cannot support the hail, it falls from the clouds and becomes the hail we see.

[Edit this paragraph] Hail characteristics

Generally speaking, hail has the following characteristics:

① Strong locality, each hail The scope of influence is generally about tens to thousands of meters wide, and about hundreds of meters to more than ten kilometers long;

②The duration is short, and a violent storm or hailstorm usually lasts only 2 to 10 minutes, and a few in More than 30 minutes;

③ Significantly affected by terrain, the more complex the terrain, the more likely it is for hail to occur;

④ Inter-annual changes are large, and in the same area, it may occur multiple times in a row in some years , the number of occurrences in some years is very small or even non-existent;

⑤ The occurrence area is wide, and it can occur in a wide range of climate zones from subtropical to temperate zones, but the occurrence frequency is mostly in temperate zones.

[Edit this paragraph] Hail classification

Hail is divided into three levels based on the diameter of the majority of hailstones (general hailstones) during a hailstorm, the accumulated hail time and the thickness of hail accumulation.

1. Light hail: the diameter of most hailstones does not exceed 0.5 cm, the cumulative hail time does not exceed 10 minutes, and the thickness of hail on the ground does not exceed 2 cm;

2. Moderate hail: the majority The diameter of the hailstone is 0.5 to 2.0 cm, the cumulative hail time is 10 to 30 minutes, and the thickness of the hail on the ground is 2 to 5 cm;

3. Heavy hail: Most hailstones are more than 2.0 cm in diameter, and the cumulative hail time is more than 30 minutes. The thickness of hail on the ground is more than 5 cm.

[Edit this paragraph] Hail hazard

Hail hazard is a severe meteorological disaster caused by strong convective weather systems. Although its range is small and its duration is relatively short, , but it is fierce and intense, and is often accompanied by bursts of disastrous weather processes such as strong winds, heavy precipitation, and rapid cooling. China is a country where hail disasters occur frequently. Hail causes huge losses to agriculture, construction, communications, electricity, transportation, and people's lives and properties every year. According to relevant statistics, the economic losses caused by hail in my country every year amount to hundreds of millions or even billions of yuan.

Many people have experienced hail during thunderstorms, usually no larger than a softball, falling from storm clouds.

However, sometimes hailstones can be quite large, with 80-pound hailstones falling from the sky. When they hit the ground, they break into many small pieces. The most mysterious thing is that huge hailstones fall vertically from the sky when there is no clouds in the sky. Many incidents have confirmed that the wings of airplanes were hit by hailstones. At present, scientists still cannot explain why such huge hailstones appear.

[Edit this paragraph] Hail prevention and control

1. Forecasting

Since the 1980s, with the development of weather radar, satellite cloud image reception, computers and communication transmission and other advanced equipment are widely used in meteorological operations, which has greatly improved the ability to track and monitor hail activity. If the local meteorological observatory (station) discovers hail weather, it will immediately report it to the potentially affected meteorological observatories and stations. 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, so that the forecast accuracy continues to improve. In order to transmit hail warning information to government leaders and the masses at all levels as early as possible, meteorological departments at all levels issue "alerts" and "emergency warnings" through local radio stations, TV stations, telephones, computer service terminals and disastrous weather warning systems and other media , enabling all sectors of society and the general public to take defensive measures in advance, avoid and reduce disaster losses, and achieve obvious social and economic benefits.

2. Prevention

my country is one of the earliest countries in the world to implement artificial hail prevention. Due to the serious hail disaster in our country, hail prevention work has received the attention and support of the government. At present, many provinces have established long-term test sites, conducted rigorous experiments, and achieved many valuable scientific research results. Carry out artificial hail prevention to make it develop in the direction expected by people and achieve the purpose of reducing disasters. Currently commonly used methods are: ① use rockets, anti-aircraft guns or aircraft to directly send catalysts such as silver iodide, lead iodide and dry ice into the clouds; ② use catalysts such as silver iodide, lead iodide and dry ice to form in cumulonimbus clouds on the ground. In the past, these substances were sent into the free atmosphere, allowing these substances to act as hail embryos in the hail cloud, causing the hail embryos to increase and the hail to become smaller; ③ Fire rockets at hail clouds on the ground to use anti-aircraft artillery, or fire rockets at hail clouds from airplanes , Drop bombs to destroy the moisture transport to the hail cloud; ④ Use rockets and anti-aircraft guns to scatter condensation nuclei on the warm cloud parts to cause the clouds to form precipitation to reduce the moisture in the clouds; Spread ice nuclei on the cold cloud parts to suppress hail embryo growth.

3. Agricultural hail prevention measures

Common methods include: ① In hail-prone areas, plant grass and trees, increase forest area, improve the landscape environment, destroy hail cloud conditions, and achieve reduction Purpose of hail disaster; ② increase crops with strong hail resistance and recovery ability; ③ harvest mature crops in time; ④ During the hail season in areas with frequent hail disasters, farmers carry hail prevention tools, such as bamboo baskets, wicker baskets, etc., with them when going to the fields to reduce personal casualties.

[Edit this paragraph] Analysis of hail disasters in China

Geographical distribution of hail disasters in China

Hail activity is not only related to weather systems, but also affected by terrain and landforms The impact is also great. Our country has a vast territory, complex topography, and great differences in landforms. Moreover, our country has the largest plateau in the world, which complicates atmospheric circulation. Therefore, hail weather in my country affects a large area, and hail disasters occur in a wide area. Based on the comparative analysis of the spatial pattern of hail disasters in China based on relevant data, we have the following four insights. (1) Hail disaster affects a wide range. Although hail disaster is a small-scale disaster event, hail disaster occurs in most areas of our country. Almost all provinces have more or less records of hail disasters. The number of affected counties is close to half of the counties in the country. This is fully It shows that the distribution of hail disaster is quite wide. (2) The distribution of hail disasters is highly discrete. Most of the hail fell in individual counties and districts. (3) The distribution of hail disasters is obviously localized. Hail disasters often occur in certain specific areas, especially the piedmont areas and agricultural areas east of the Qinghai-Tibet Plateau, which is closely related to the conditions for the formation of hail disasters. (4) The overall distribution pattern of hail disasters in China is more in the central and eastern parts and less in the west. The spatial distribution shows a pattern of one region, two belts, and seven centers (Figure 3). One of the areas refers to the area north of the Yangtze River in my country, south of the Yanshan Mountains, and east of the Qinghai-Tibet Plateau. It is the area prone to hail disasters in China; the two areas refer to the hail disaster-prone areas on the outer edge of China's first ladder (especially the east area) and the eastern edge of the second step and the hail disaster-prone areas to the east are the hail disaster-prone zones in China; the seven centers refer to several hail disaster-prone centers scattered in the two hail-prone belts: Northeast high-value area, North China high-value area, Hubei Henan high-value area, Nanling high-value area, eastern Sichuan, Hubei, western Hunan high-value area, Gansu-Qingdong high-value area, Kashgar Aksu high-value area.

The temporal distribution of hail disasters in China

Generally speaking, the temporal distribution of hail disasters in China is very wide. Although hail can fall at any time within a day, there is a relatively concentrated hail period in various regions across the country. Analysis of relevant data shows that 70% of the hailstorm time in most areas of my country is concentrated between 13:00 and 19:00 local time, with the largest number occurring between 14:00 and 16:00. Hail falls mostly at night in western Hunan, Sichuan Basin, and western Hubei, and in some places on the Qinghai-Tibet Plateau, hail falls mostly at noon. In addition, hailstorms in various parts of my country also have obvious monthly changes, which are consistent with the monthly changes in atmospheric circulation and monsoon climate characteristics. The hailstorm area moves northward with the northward movement of the South Branch jet stream, and the arrival of hailstorms in various regions is faster than that of the rain belt. Arriving about a month early.

Generally speaking, it is from March to April in Fujian, Guangdong, Guangxi, Hainan, and Taiwan, from March to August in Jiangxi, Zhejiang, Jiangsu, and Shanghai, from April to May in parts of Hunan, Guizhou, Yunnan, and Xinjiang, and from April to May in Qinling Mountains. , Huaihe River in most areas from April to August, North China and parts of Tibet from May to September, Shanxi, Shaanxi, Ningxia and other areas from June to August, the vast northern areas from June to July, the Qinghai-Tibet Plateau and other areas In high mountain areas, hail is the most common month from June to September (Figure 4). In addition, because hail is very localized, there are great interannual variations in various regions and even across the country.

Analysis of the disaster-producing environment and disaster-causing factors of hail in China

From the perspective of regional natural disaster system theory, hail disaster is the interaction between the disaster-producing environment of hail, disaster-causing factors, and disaster-stricken bodies. Disasters caused by the effects. Hail and heavy rain are both strong convective weather processes, which are restricted by terrain and often occur together. Therefore, heavy rain and terrain have become the main factors in the disaster-producing environment of hail disasters. The intensity and regional differentiation of hail disasters first depend on the characteristics of hail. From the perspective of regional differentiation of hail in my country, the high-value hail area shows the characteristics of one zone and two zones: the first zone refers to the hail-prone area of ??the Qinghai-Tibet Plateau; the two zones refer to the direction. Hail-prone belt and northern hail-prone belt. The former is mainly distributed in the Yunnan-Guizhou Plateau at an altitude of 1,000 to 2,000 meters, extending eastward to the borders of western Hunan and Sichuan and Hubei. The latter starts from the northern part of the Qinghai-Tibet Plateau from the Qilian Mountains and Liupan Mountains through the Loess Plateau and the Inner Mongolia Plateau. connect. Compared with the regional differentiation of hail damage (hail) in China, the regional differentiation of hail damage has an obvious trend of expanding eastward, southward, and westward, with the following three obvious differences. First, from a macro-regional perspective, hail disaster-prone areas are completely different from hail disaster-prone areas with the highest frequency. The former is the densely populated North China-middle and lower reaches of the Yangtze River, while the latter is the sparsely populated Qinghai-Tibet Plateau region. Second, there are two frequent hail damage and disaster zones, but the former is located further east than the latter, especially in the east, forming a north-south hail disaster zone. Third, the hail-prone areas are located in hail-prone belts and are distributed in a lumpy shape. It can be seen that the regional differentiation of hail disasters in my country is deeply affected by the scope of human activities, showing a spatial pattern with more hail in the central and eastern parts and less in the west. From the comparison of the spatial differentiation of hail and hail disasters in the region, hail is just a natural process. The changes in the nature of the disaster-stricken bodies mean that the high-value areas caused by hail are not necessarily the high-value disaster areas. Although the disaster-stricken body is not the direct driving force of the disaster, it makes the hail disaster relatively expand or reduce.

Analysis of victims of hail disasters in China

The regional differentiation of hail disasters in China is deeply affected by the victims. By classifying and analyzing the existing hail cases in China one by one, The research results show that there are 6 major categories and 20 subcategories of main types of victims of hail disasters in my country, among which food crops are affected the most. From the perspective of dynamic changes, the following four subcategories are worth noting: First, corn, the ranking of disasters (compared to other crops) has increased, which is related to the extensiveness of corn planting in my country and the development of mulched corn planting. Using mulch to advance the growth period of crops will undoubtedly increase the period of time when hail disasters occur. The second is cotton. The number of disasters has increased significantly, especially in some major cotton growing areas. It can be seen that changes in crop varieties and crop areas directly affect the amplification or reduction of disasters. Third, the damage to vegetables, fruits, and flowers has increased. With the improvement of urbanization level, the development of vegetables, fruits, and fruits, especially flowers, in urban fringe areas, coupled with the widespread use of greenhouse technology, has made them more susceptible to hail disasters. The probability increases. It can be seen that changes in land economic crop output directly affect the vulnerability of disaster-stricken bodies. Fourth, the number of communications disasters has increased sharply. With the rapid development of the country's communications industry, especially the rise of the Internet in recent years, the vulnerability of hail victims has been magnified.

The area with the most hail in China

The area with the most hail in China is the Qinghai-Tibet Plateau, such as Heihe (Nagqu) in northeastern Tibet, with an average of 35.9 days of hail every year (the maximum number of years has dropped to 53 days, at least 23 days); followed by Bangor 31.4 days, Shenzha 28.0 days, Amdo 27.9 days, Suoxian 27.6 days, all occur on the Qinghai-Tibet Plateau.