From square kilometre to house roof
The hailstone size MESHS refers to the maximum hailstone within one square kilometre. However, the areas considered in practice are generally smaller: A single-family house, for example, might have a roof area of about 100 square metres, which is 10,000 times smaller than the reference area of MESHS (see Figure 2). From the user's point of view, MESHS is therefore not easy to apply.
In order to be able to estimate the actual hailstone size occurring on the scale of a house roof, a conversion was implemented. LEHA is derived from MESHS, which stands for "Largest Expected Hail on a reference Area". The development and validation of the conversion was supported by the hail reports from the MeteoSwiss app and data from the hail measurement network, as well as by case studies of damage events by the project partners.
The choice of reference area for LEHA is determined by the respective application. The transformation is done with a calculation rule and the LEHA diagram (see Figure 3). This derived quantity is particularly relevant for applications relating to damage to buildings and vehicles.