During the period 1975-1990, a part of Baranya later Bacs-Kiskun county of about 1500km2 of area was covered by a rocket hail suppression system (BRJE and BaRJE), operated by OMSZ (Hungarian Meteorological Service). The high costs, combined with Állami Biztosító (National Insurance) refusing further financial support, forced OMSZ to shut down this system.
However an ever-growing demand from farmers and agricultural estates rose for a system providing effective protection against hailstorms in an even larger region. The NEFELA Association for Hail Suppression in Southern Hungary
was founded in 1991, with the co-operation of large agricultural estates of Baranya, Somogy and Tolna county, the financial support of ÁB-Aegon Rt. and Hungária Biztosító Rt. (insurance companies) and the state property provided by OMSZ.
The main decision-making body of the Association is the Members’ Meeting. Members are representatives of the founders and other companies that joined the Association after its founding. Each newly joining company is allowed one representative in the Members’ Meeting. The Meeting is held once a year.
The Association’s permanent staff has 4 members. During the summer 8-10 meteorologists are employed additionally. The ground generators of the Association are operated by 141 commissioned operators.
2. Vortex type ground generators
The ground generator hail suppression system was installed in 1991 with the valuable support of the French researcher Prof. J. Dessens, who had over 30 years of technical and methodological experience. We manufactured our equipment based on an instrument acquired from the French Association for Hail Suppression ANELFA.
The three main parts of the instrument are: air container (1), solution container (2), burner chimney (3). The air container provides 1.3 bar overpressure in the solution container, which is filled with a silver iodide–acetone solution of concentration 8 g/l. The solution is vaporized through a 30µm nozzle into the burner chimney, where it burns at a temperature of about 800-900 degrees Celsius. Abrupt cooling makes the escaped AgI molecules form crystals of size 0.06–0.08µm, the structure of which are similar to that of ice crystals.
One gram of AgI produces approx. 1014–1015 active crystal particles. The amount of solution burnt hourly depends on the values of overpressure and nozzle size applied; in our case it is 0.8–1.0 litres.
3. Basic principles of ground generator hail suppression
In nature, ice droplets form in high altitudes, at temperature of -10oC to -15oC. First a small number of larger heavily subcooled water droplets freeze, and then begin to rapidly grow by collection liquid droplets. As a consequence large hailstones develop, which ascending air motion (convection) cannot hold, so that they fall with high velocity and reach the ground in a very short time.
During hail suppression, silver iodide (AgI) is injected into the cloud layer by the generators on the ground. To be effective, AgI particles need to be present in the thundercloud in high concentration and prior to the development of hailstones. The convection (warm air current upwards) that helps formation of thunderclouds, and the ascendance of air ahead of cold fronts, allow AgI particles to get into the cloud. Once in there, the AgI crystals (having similar structure to that of ice crystals) facilitate the freezing of subcooled water droplets, hence producing a larger number of initiatory hailstones, which concurrently collect liquid droplets. Artificial intervention in hail development results in more and smaller hailstones. The smaller hailstones fall with a lower velocity and thus spend more time in the positive temperature domain. Melting further decreases their size, and the damage caused by hailstones will be diminished.
The mechanism of precipitation forming within thunderclouds does not allow to completely preventing hailstorms that may cause substantial damage. The ground generator hail suppression system will fail if a rapidly moving thundercloud reaches the covered region with hailstones fully developed. This makes the expansion of the system and cooperation with neighbouring countries reasonable.
4. Operating the system
Currently the Association operates 141 ground generators in three counties: 44 in Baranya, 60 in Somogy, 37 in Tolna (see map). An optimal network was intended during installation, with a desirable distance of 10km between neighbouring generators. One qualified operator operates each instrument. Direct access to operators is established through cellular phones supplied by the Association.
The generator network is controlled at the radar station at Hármas-hegy, where meteorologists are on 24-hour service from May 1st till September 30th.
The radar station is equipped with an MRL-5 type, two-channel, high definition meteorological radar of range 256km, making visible all precipitate clouds both in PPI (mapping) and RHI (vertical) mode. This radar makes it possible to assume the danger, development tendency, intensity, velocity and direction etc. from large distance. A METEOSAT satellite receiver and weather prediction maps facilitate the meteorologists’ work. These devices, together with the expert team, allow the service to learn about every meteorological event that would affect the region.
After data evaluation the meteorologist on service makes a decision about where and for how long the generators should be operated. Interventions should be initiated at least two hours before thunderstorms (approaching fronts). There is an average of 40–50 interventions a year; generators operate for 2–10 hours at a time, which sums up to an average of 200–260 hours annually. Generally all generators are operated simultaneously, but switching on only the Eastern or only the Western half of the network, depending on the weather conditions, is possible.
The network co-workers fulfill the refilling of generators, as well as reparation, maintenance and testing tasks. Generators are transported to the depot of the Association after the preventive season. Reparation, maintenance, and testing takes place in winter. The transportation to the operators begins before the preventive season.
Financial background of the Association
The system is financially supported by farmers and vine-growing communities of the covered region, insurance companies, local governments, and (providing 39% of the total income in 2006) the FVM (Ministry of Agriculture).
The future of hail suppression depends on all those affected by hailstorms. With cooperation and joining the Association, the system’s maintenance will be more easily financed.
Budget of the Association and the members’ annual contribution are established by the Members’ Meeting, in which each member has one representative.
Pécs, January 12th, 2007.
Compiled by Bereczki Károly meteorologist
Budapest, January 23th, 2007.
Translated by Birszki Bálint