Modifying Hail In Bavaria



The region of the Bavarian Plains, situated at the northern slope of the Alps is frequently affected by hail damage. Therefore, the Bayerischer Landtag (Bavarian State Parliament) decided to start an experiment using silver iodide released from rockets, as well as from ground generators, to obtain more information about possibilities of suppressing hail by seeding hail clouds. Silver iodide seeding possibly may cause hail suppression by adding artificial ice nuclei to an air mass in which natural ice nuclei are very few in number, as is known for tropical air masses in Central Europe. If it were possible to generate a large number of small ice particles instead of a small number of big hailstones, damage could be reduced considerably. But there is still some doubt whether the usual seeding action will get a sufficient number of artificial ice nuclei into sensitive parts of the thunderstorm cloud at the right time.

The region of Rosenheim was chosen for the experimental area due to the special interest in this field shown by the Rosenheim local authorities as well as the farmers’ association. The main part of the Rosenheim district consists of plains with only small hills and a number of minor lakes. The southern part is on the northern slope of the Alps and extends to the border of Austria (Tyrol). The Rosenheim district covers an area of approximately 320 square miles, extending 21 miles from north to south along the Inn River, and 15 miles west to east. Lake Chiemsee forms part of the eastern border. The intention was to suppress hail as much as possible.

Organization of the experiment 

Seventy six rocket posts were installed along the Inn River and manned on a voluntary basis. They are stationed in a triple chain, two series west of the Inn and one on the eastern border with a mean distance of one to two miles from post to post. Some posts are situated where local thunderstorms are especially frequent. Each post has up to ten rockets at its disposal. They are stored according to state regulations for storing explosives. The two men at each post have taken a course in handling explosives and a special examination. The rockets each contain 800 gms of cheddite and 16 gms of AgI. They can attain a maximum height of about 4500 ft above the ground. Since the freezing level in summer over Bavaria is about 10,000 to 12,000 ft, the seeding agent must be transported to its working height by the updraft normally connected with thunderstorms and must be dissipated by turbulence. It is quite possible that the wind distribution between the height of the exploding rocket and the place where seeding must be done to be effective may be unfavorable. This risk cannot be excluded. The rocket posts are alerted normally by radio and in special cases by telephone…

…In addition to the rocket network, a network of silver iodide ground generators was installed. Thirty propane gas generators (eighteen in the first experimental year) dispersing 1 gm of AgI per minute dissolved with Nal in acetone were distributed in a double chain west of the target area. The upwind situation was chosen to give the seeding agent produced near the ground more time to reach its working height by updraft and turbulent mixing with the surrounding air. The ground generator posts are alerted in the same way as the rocket posts but, for the ground generators, the time of operation is announced by broadcast so that, normally, all generators are in action during the same time…

…During the eight year seeding period, damage occurred on 72 per cent of the seeded days, and on 100 per cent during the preseeding period. Looking at the same table for the windward situated control area of Ebersberg, Aibling, Miesbach, Wasserburg, one finds also that in control area A the number of hail damage days during the experimental period from 1958 to 1965 was only 71 per cent compared to 100 per cent during the period from 1950 to 1957. Thus, one could conclude that the eight year period from 1958 to 1965 had less hail damage generally than the period from 1950 to 1957 and therefore no effect of seeding appears to exist.



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