The hail suppression comprises measures by which the occurrence of local hail at storm with various methods of weather modification is to be prevented or reduced. In this way, agriculture and the population are to be partially protected from the damage caused by hailstorms. The case most frequently used method is the introduction of silver iodide - acetone mixture into the clouds by hail airmen and hail rockets.
Origin and historical methods
Since ancient times people tried to ward off the damage caused by hailstorms by apotropaic actions . Seneca already describes in his Naturales quaestiones permanently employed "hail guards" in the city of Kleonai , who warned of impending hail and induced the population to sacrifice animals and blood in order to stop the storm.
In its early days, the Christian Church attributed hailstorms to the work of demons , which they had to fight through prayer, Holy Mass or consecrated water . From the 9th century onwards, crucifixes , signs of the cross and formulas were used as incantation rites to banish demonic storms in the wilderness. The parchment of the Seckau hail evocation dates from the 12th century. Many local customs were originally of pagan origin and were only temporarily tolerated by the church or reinterpreted by the church, such as the sacrificial custom of the "hail cattle", which were given away by village communities to monasteries to ask for their assistance against hailstorms. Hail crosses set up on paths have been known since the 13th century, and hail processions and palm sticks were common Christian forms of hail defense. In Europe, the tradition of weather ringing has also been documented since at least the 15th century , which was later supplemented by "weather shooting" to drive away the hail with noise. Maria Theresa forbade this custom in the 18th century, because "the vault [...] is finally being driven on the neighbors with even greater force". In other areas as well, such customs were restricted or forbidden in the course of the Enlightenment . In particular, the worship of weather saints was widely practiced until the 19th and 20th centuries.
Hail shooting using cannons and rockets was accepted as an object of scientific research in the Enlightenment and was analyzed on the basis of the knowledge available at the time. In 1788, for example, Placidus Heinrich wrote a treatise “About the effect of guns on thunderclouds”, which tried to evaluate various methods of weather shooting using methods and knowledge of science known at the time and recorded numerous reports from different parts of the Alpine region. Hail shooting was increasingly taken up at the end of the 19th century. In 1900 in France, Italy and Austria-Hungary around 15,000 weather shooting systems were supposed to have been installed. In particular, soot particles and later silver iodide were now shot as crystal nuclei.
Cloud inoculation with silver iodide
Hailstones form in water-rich, high-reaching clouds with strong up and down winds. When it collides with supercooled water droplets, layers of ice accumulate around a crystallization core, creating a hailstones with a shell-like structure. With the so-called cloud inoculation, artificially introduced crystallization nuclei are supposed to lead to an increased formation of hailstones, which thus remain smaller and cause less damage on impact. The same principle should also lead to the formation of larger raindrops, so that there is increased and earlier rainfall, so the formation of hail should even be completely avoidable and the amount of precipitation increased in dry areas. The principle goes back to the American Nobel Prize winner Irving Langmuir , who developed it in the 1940s with Vincent Schaefer and Bernard Vonnegut .
Assessment of applicability and effectiveness
While an effect is theoretically conceivable and justifiable, in practice there are major problems with all methods used in the actual introduction of silver iodide into suitable cloud layers at the right time, since the local weather conditions fluctuate greatly. So far there are no scientific studies to measure success, and no clear results can be derived from operational studies. For the increased, targeted rain down by means of cloud inoculation, only a local effect of 10% change in the amount of precipitation could be statistically proven. Israeli studies question whether the method is more effective. In the case of larger storm clouds (approx. 3 km in diameter), even with careful estimates, at least 2 ⋅ 10 18 drops must be assumed, which could potentially form hail. With an assumed generation of 2 10 13 condensation nuclei per gram of silver iodide used, even if several hundred kilograms of the substance are distributed, there are hardly enough nuclei to produce a greater effect.
Accordingly, announced attempts to use such a technique to bring certain rain clouds to rain down the water content ( artificial rain ) in order to make certain major events rainproof, such as the May 9th parades in Moscow in 2005 and 2008 or the arrival of the Olympic one, often failed Feuers in Beijing 2008. The reported successes are controversial. Russian meteorologists announced that they wanted to prevent rain at the G8 summit in Saint Petersburg in 2006 using silver iodide, but there were downpours during the conference.
Hail planes are specially equipped planes that deploy the finest silver iodide particles under the cloud base. Updrafts are supposed to transport these particles into the thundercloud. The particles are produced with "torches" containing pressed black powder with approx. 7% silver iodide or by burning a silver iodide-acetone solution in special generators.
The majority of German meteorologists doubt the effectiveness of the method, for which no evidence has yet been provided. The Austrian Central Institute for Meteorology and Geodynamics carried out a long-term study between 1981 and 2000. In some cases, the evaluations show a hail damage reduction of up to 40%, but no data from unaffected regions were available as a comparison value. A six-year study with Bavarian hail planes carried out by the German Aerospace Center came to the conclusion in 1993 that an effect could not be scientifically proven. A study carried out by ETH Zurich with US scientists in the 1980s came to the conclusion that the method “did not work”.
Use in Germany
In Germany, the Bavarian districts of Rosenheim , Traunstein and Miesbach jointly operate hail planes stationed at Vogtareuth airfield . The annual costs of 200,000 euros are shared, but the relay is operated by the Rosenheim district. The squadron consists of two twin-engine aircraft that work with a solution of silver iodide and acetone. The Rems-Murr district and the Schwarzwald-Baar district in Baden-Württemberg also each have two appropriately equipped aircraft. In the Ortenaukreis an association is a carrier of a hail plane. In Rhineland-Palatinate, a wine cooperative operates a hail plane. Machines are also stationed at Stuttgart Airport that are not only used in agriculture, but are also intended to protect the cars from the Mercedes-Benz works that are ready for delivery from hail damage. These five hail planes cause annual costs of around 350,000 euros.
Use in Switzerland
Hail rockets and hail shooting
In addition to the hail planes, rockets are also used to introduce condensation nuclei into potential hail clouds. This technique goes back to the 18th century, when the force of the detonation was supposed to serve to disperse the clouds. These methods proved to be ineffective in the studies carried out for this purpose, but were still carried out well into the 20th century. Since the late 1940s, hail rockets have contained silver iodide, which is released when the rocket explodes. Depending on the model, the rockets reached altitudes of 1,500 m to 14,000 m. Hail shooting was banned in Germany in 1933, and the rearmament prohibition made it very difficult to resume using the new silver iodide rockets after the war. In the Inntal , programs based on the model of Swiss hail defense were started in the 1950s, although Swiss studies had not found any significant effect of hail defense through rocket fire. The rocket tests were continued in Switzerland, but the effectiveness of the measures remained controversial there too. Lately the number of hailers dropped, in 2006 around 1400 hailers shot down a total of 2500 hail rockets a year in Switzerland - who took compulsory exams and refresher courses. The shooters were organized in the Swiss Association for Hail Control (SVH) until the end of 2019. The association was dissolved because two of the last three remaining sub-associations had dissolved in 2017, including the Bernese-Solothurn hail defense association. The Mittelland-Emmental hail defense association was dissolved at the end of 2016. Today only the Hail Defense Association of Eastern Switzerland (HAVOS) and one association in Western Switzerland exist.
Like hail rockets, hail cannons were also used to bombard thunderclouds with normal projectiles until the beginning of the 20th century. More recently, however, sonic cannons have been developed that rely on a different potential mechanism of action. The sound waves generated by propane gas explosions are intended to mix the air layers through and prevent ice from forming on dirt particles and cause rain to fall instead. The American Society of Civil Engineers considers hail cannons to be ineffective, the devices do not generate frequencies that are not generated (to a greater extent) by natural thunder, and the range of the sound waves is also too short to have any influence on the formation of hailstones.
As passive methods of hail defense, hail protection nets are mainly used, which cannot prevent the occurrence of hail, but can prevent damage to buildings and objects caused by hailstorms. However, due to the size of the area and the associated costs, nets are hardly an option for use to protect agricultural land. The nets are stretched over the entire plants in the form of a gable roof and let the hailstones fall down in the eaves area. The nets are made of polyethylene (PE) and come in a wide variety of colors. PVC has not proven its worth, as hydrogen chloride is split off under the influence of UV , which makes the plastic brittle . The durability of the nets is five to eight years, depending on the additives ( pigments , soot, UV stabilizers). The advantages of hail protection nets are that they are completely covered and the fruit can be protected against sunburn damage. The reduction in the supply of light can have a disadvantage, as it can lead to poor color development in fruits and delayed ripening.
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