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