Hail

Summary of hail

What is hail?

Hail, also known as sleet, is commonly known as hail,called “hailstone” in some areas. It is the solid precipitation falling to the ground from the strongly developing cumulonimbus clouds, small as beans, big as eggs and fists. It is a kind of meteorological disaster that mainly damages and destroys ground objects. It does great harm to agriculture and is one of the serious disasters in China.

Classification of hail

According to the diameter, cumulative time and thickness of most hail (general hail) during a hail fall, hail is classified into three grades.

Light hailMost hail are less than 0.5 cm in diameter, no more than 10 min in cumulative hail time and no more than 2 cm in surface area.

Moderate hailMost hail have a diameter of 0.5 to 2.0 cm, a cumulative hail time of 10 to 30 min, and a thickness of 2 to 5 cm.

Heavy hailMost hail have diameters of more than 2.0 cm, cumulative hail time of more than 30 minutes and hail thickness of more than 5 cm.

Hail hazards

Hail disaster is a severe meteorological disaster caused by severe convective weather system, although its scope is small and time is short, it has a strong and intense coming force, and often accompanied by paroxysmal disastrous weather processes such as strong wind, heavy rainfall and sharp cooling.China is a country with frequent hail disasters,every year, hail brings huge losses to agriculture, construction, communications, electricity, transportation and people's lives and property. According to statistics, the economic losses caused by hail in China are hundreds of millions of yuan or even billions of yuan every year.

Prevention and control of hail

Early warning signals of hail

Early warning signals of hail are divided into two levels, which are orange and red, respectively, as shown in Table 6-8.

Key points of hail avoidance

(1) Close doors and windows, and properly place outdoor articles that are vulnerable to hail.

(2) Do not go out，ensure the elderly and children stay at home.

(3) Suspend outdoor activities, such as in the outdoors, do not avoid hail under the eaves of high-rise buildings, chimneys, poles or trees.

(4)While preventing hail, we should also be prepared for lightning protection.

How to reduce the hail damage?

(1) Strengthen weather forecast and reduce hail loss.

(2)Grasp the hail-prone time, adjust the planting time and planting structure. For example, we should select root crops such as potatoes, peanuts and sweet potatoes to reduce hail disaster.

(3) Utilize high-tech means to dispel and eliminate hail artificially, when hail clouds occur, hail suppression operations can be carried out in time.

(4)After the hail disaster, if the affected crops can resume their growth, should immediately increase fertility, increase ground temperature, or use water irrigation to make hail melt away as soon as possible, so as to reduce the loss caused by hail.

Agricultural hail suppression measures

(1) Take favorable agrotechnical measures to prevent hail. According to the season in which hail occurs, crops are harvested to avoid the hazards of hail. If wheat in harvest period is vulnerable to hail disaster, early sowing or planting early-maturing varieties should be considered in planting measures so as to make it harvest ahead of time and avoid hail season.When such measures are still unavoidable, crop cultivation should be adjusted.Hard stalk crops, such as maize and millet, should be selected to grow in areas where hail is more frequent. Although these crops are damaged by hail, they are easy to resume growth.After crop disaster, field management should be strengthened, such as timely soil cultivation, middle tillage, watering and fertilization, and meticulous management, so as to restore crop growth as soon as possible.

(2) Changing the local climate and environment to prevent hail and vigorously planting trees are good ways to reduce the hazards of hail.For example, in the southwest mountainous areas of Yunnan and Guangxi, hail hazards were more in the past, due to vigorous afforestation and climate regulation in recent years, hail was greatly reduced. Improvement of bare saline-alkali land is also a good way to reduce hail.

(3) Artificial methods to prevent hail.This method can be divided into fire prevention and eliminate.According to weather forecast and people's experience, when hailstorms are coming, hail suppression shed or hail suppression cover will be erected on focused crops or key crops to protect them from hazards,the effect is remarkable, but the cost of labor and materials is a major deficiency.

“Elimination” is the elimination of hail by artificial means with homemade cannon and rockets,which has been widely used in all parts of our country and has achieved certain results.Under the condition of modern science and technology, catalysts such as silver, lead iodide and dry ice are directly sent to the clouds by rockets, anti-aircraft guns or airplanes to destroy the water transport to hail clouds and inhibit the growth of hail embryos.