The Relationship Between Hailfall and Damage to Buildings

On July 21, 1998 a severe hailstorm hit the city of Lucerne and its agglomeration (Lucerne-storm). The storm initiated damage on buildings in the order of 70 Million Swiss Francs. The event was observed by the research radar at the Institute for Atmospheric Science (ETH-Zurich) and the intensities of the hailfall were measured. In earlier studies, good relationships between radar measurements of hail and the corresponding damage on agriculture (e.g. wheat, corn) have already been established. Hence, the question arises whether a good relationship can also be obtained comparing radar-derived measurements and hail damage on buildings. So far, due to the lack of damage data, this question has not been tackled before.

The presented study deals with the mentioned problem using data from the Lucerne-storm. Initially, the total kinetic hail energy, produced at one spot by the storm, had to be computed from the radar-derived measurements with a fine spatial resolution. The obtained energy values were related to the damage on buildings. Furthermore, it was necessary to collect data from the damaged buildings, which appeared to be a major effort, since the data have to be extracted from the damage report of each building. In addition, the spatial location of the building was determined, in order to relate spatially both values (radar and building). However, the established database is still small (with 230 objects). Therefore, the presented results should be thought as first hints about the possibility to estimate the amount of hail damage on buildings by using radar measurements.

Using only a stratification according to the insured value of the entire building or main components of a building (roof, facade), no direct relationship is obtained. The damage has to be further stratified according to the hit material of the building components. Roof materials (like tiles or "eternit"-plates) or facade materials (like plaster or paint), that cover a large part of the buildings surface, show the best relationship between the kinetic hail energy and the particular damage. Large differences can be found depending on whether the object was hit in the part of the storm with severe wind gusts or in a more calm part; buildings showing over proportional damage were all situated in the "severe wind zone" of the storm. On the contrary, materials, which do not cover a large surface but rather appear as individual elements on the roof (like sheet metal covers, roof windows) or of the facade (like window shades or metal pipes for rain water) are often completely replaced of being repaired. As a result, the amount of damage does not depend on storm severity and no clear increase of the damage extent with increasing kinetic hail energy is obtained. The "surface"-material should be weighted with a factor representing somehow those building elements in order to get the best estimation.

A comparison between the Lucerne-storm and a stronger as well as a weaker storm is presented at the end of this study. The stronger storm occurred on August 21, 1992 and passed the city of Lucerne only a few kilometers towards the north. Severe damage occurred mainly in agricultural areas and in forest stands on Mount Rigi. The weaker storm of June 2, 1999 hit two villages in the northern part of Canton Lucerne. Some minor damages on buildings were collected within the framework of this study (about 40 objects) and compared with radar measurements. The values can successfully be placed into the relations obtained by the data of the Lucerne-storm.

It is recommended to refine the present database (register the amount of damage for every hit material - so far the integral amount was used) and to collect more objects. The recalculated relationships should then be verified on data obtained from a further independent storm, preferably with a similar severity as the Lucerne-storm.

You can also download a copy of the following: "Report: The Relationship between Hailfall and Damage to Buildings (in German)".