Hail

Hail is a form of precipitation that consists of lumps of ice and occurs mainly in warm seasons and areas. To distinguish one speaks only at a diameter of about 0.5 centimeters of hail or Eishagel, including of sleet. In aggregates of snowflakes with a diameter less than one millimeter is called Griesel.

Etymology

The common name also CASTLE for a hailstone is eponymous for the color name white as snow ( dissimilation of closed white), so what " white as a hailstone " mean and refer to a bright, pale shade of white.

Formation

Hailstones occur in the lower layers of storm clouds or within a thunderstorm cell by subcooled water, which freezes into ice on nuclei. These cores have to occur in comparatively small numbers, so that each core to the amount of water available is sufficiently large to allow a rapid growth. Since this is so is very water-rich clouds that phase transformations implemented latent heat quantities have a strong instability of the temperature stratification within the cloud result. The resulting generated strong updrafts of well 20 to 30 m / s are another important factor for the formation of hail, because the Gefrierungsprozesse have a steady increase in mass of the particles result. Without an updraft the particles would fall by gravity to remove themselves from the cloud, and thereby can not continue to grow.

It is evident here that the updraft varies greatly within the cloud, and thereby particle can pass through a circuit. First, they are lifted by the updraft, then they fall back into deeper layers of the atmosphere, take more water to be again torn up, and additional water freezes at. This process is repeated until a hailstone is too heavy to be carried by the updrafts. Can therefore be concluded that the wind strength within the storm cloud of the size of the hailstones, which is used in the inversion also for forecasting hail diameters.

The gradual emergence of the hailstones can be read off from the individual attachment layers that make up a hailstone. The bright layers indicate a very water-rich environment with correspondingly rapid freezing, while the dull areas go back to lower water contents. The cloud itself is caused by numerous small air pockets. Is the hailstone ultimately too heavy and falls out of the updraft, so it does not come to melting due to the size of the hailstone and a temperature of usually below 0 ° C.

Dissemination

Worldwide, the Middle latitudes of hail events are most affected, particularly in Central and Southern Europe, the USA, Mexico, East China, Argentina, South Africa and South-East Australia. However, the equatorial regions are affected in individual cases as appropriate soil conditions, including Kenya (especially the Kericho region ) or Ethiopia. Also on mountain edges occurs more frequently hail, for example, to the Andes in Peru, Ecuador and Colombia or the Himalayas in northern India or Nepal.

Size and fall velocity

The size of hailstones varies greatly: The hail damage scale of the Swiss Association of Cantonal Fire Insurance eleven different intensity classes, ranging from small hail less than 0.5 cm to exceptionally large hailstones of about 10 cm diameter. The weight of the grains varies accordingly between 0.1 g and more than 0.5 kg. Damage to cars, glass, and tents it comes from a diameter of about 2 cm; Hailstones of this size reach fall velocities of about 70 km / h; smaller hail falls more slowly ( 35 km / h), on the other hand exceptionally large can reach speeds of over 150 km / h. According to CNN, the largest hailstone in U.S. history with 20 cm diameter and a weight of 875 g on August 9, 2010, found in the American state of South Dakota.

Hail Damage

Hailstorms can cause considerable damage to plants and animals but also to buildings and cars. The majority of agricultural land is exposed without protection to the hail. In addition, hailstorms cause high damage to trees and thus reduce the forest resources. Due to the small size of hailstorms only limited damage are usually recorded. The hailstorm with the highest amount of damage was performed in 1984 in Munich. The damage amounted to € 1.5 billion.

The existential circumstances financial risk of crop failure due to hail can be secured only by special insurance (hail insurance). The hail insurance is a form of life insurance where the insured ground products, especially all arable crops, are against any damage caused by the action of hailstorms, insured. Also greenhouses may be included. The insurance of buildings, vehicles and the like is not possible under the hail insurance.

Due to substantial hail damage has already been attempted quite early to develop ways and means to prevent hail.

Frequency

Climate change seems to have an influence on the frequency of occurrence of hail storms. As researchers at the University of Karlsruhe found out the number of hail storms has increased markedly in recent decades. Have the number of days, nor located with hail damage in 1986 at five, it had grown to 34 in 2004.

Hail suppression

Throughout the 19th century to ward off the threat of thunderstorms the "Weather Ring". At the turn of the 20th century, it was partially replaced by gun salutes.

Promising only was the discovery that silver iodide ( AgI ) is eiskeimbildend already at -15.2 ° C and is also regarded as environmentally friendly, whereas water solidifies without crystallization nucleus only at about -40 ° C to ice. From the mid- 1950s they began to shoot silver iodide rockets with hail in thunderclouds. Today, in part, the hail suppression pilots do the job with their aircraft. The mounted to the aircraft generators are used for fast evaporation of the electrons released at the cloud base silver iodide -acetone solution, thereby forming AgI crystals and are carried into the storm cloud in the result from the thermal. It is shown here that only one gram of AgI approximately 1013 particles result, the number of naturally occurring condensation nuclei is thus greatly increased artificially. Thus the excess moisture supply can be distributed finer and the formation of extremely large hailstones is prohibited. In addition, the likelihood that the smaller hailstones melt during the Ausregnens the thundercloud and just more than heavy drops hit the ground. The effectiveness of hail fighting with silver iodide, however, is not scientifically proven and there have also been observed hail reinforcements despite vaccination. Because the processes are very complex in a thunderstorm cell, is the proof of whether, for example, would have fallen even without vaccination no hail, very difficult to establish.

Recently, anti-hail nets are used in agriculture. These are clamped in gable roof form over the entire plant and let the hailstones fall down in the eaves area. The nets are made of polyethylene (PE) and have a variety of colors, PVC has not been successful, because under the influence of UV chlorides are eliminated. The durability of the networks range from 5-8 years, depending on admixtures (pigments, Rußbeimengungen, UV stabilizers ). The benefits of anti-hail nets are the complete protection of fruit and wood, the regular harvests and additional protection against sunburn damage to the fruit. A disadvantage, however, is the reduction of Lichtdargebots, which is associated with a poorer color training with fruits and a delay in maturity. Also, temperature and humidity under the networks are a little lower, which increases a longer leaf wetness after rainfall caused and thus the susceptibility to plant diseases (especially fungal diseases). In addition, anti-hail nets are very expensive and therefore only for special crops (eg fruit ) profitable. Modern anti-hail nets can be opened in order to avoid fracture under stress due to snow.

Another attempt is made with hail cannons (English Hail cannon ). They usually consist of a small ISO container with attached funnel. At intervals of five seconds -energy shock waves are launched into the sky with a loud bang. In order for the hail to be converted to wet snow. When used you have to wear hearing protection.

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