CHHC: Swiss Hail Climatology

The data contain the monthly and yearly number of hail days, the monthly and yearly maximum hailstone sizes, the long-term average number of hail days per year and month of the summer half-year (April to September), per square kilometer, for Switzerland. Due to the infrequent occurrence of hail, averaging of hailstone sizes over many years is not meaningful. The long term average numbers of days with hailstone sizes of at least 2 cm and 4 cm are provided as climatology.

Hail cannot be measured on the ground over a wide area. The size is therefore derived from radar measurements. A hail day is defined as a day on which a high probability of hail on the ground was derived from the radar parameter Probability Of Hail POH. The number of hail days of at least 2 cm and 4 cm, respectively is based on radar parameter Maximum Expected Severe Hailstone Sizes (MESHS). The 24 hours between 06 UTC and 06 UTC of the following day are considered.

The return period data contain the return values of the radar-derived maximum expected hailstone sizes over a square kilometer (MESHS) and the application-focused largest expected hail stone size over a reference area of 100 square meters (LEHA), respectively, as a function of the return period T. They describe the hailstone size that is exceeded with a probability of 1/T per year under today's climate conditions, at a fixed location. The return values were determined using a stochastic resampling approach based on observed radar hail footprint data of the period 2002-2020.

Hailstone sizes MESHS are given in classes from 2 cm to greater than 6 cm in 0.5 cm increments, with the lower end being inclusive and the upper end exclusive: the grid value 2 represents values from 2 cm (incl.) to 2.5 cm (excl.).

Hailstone sizes LEHA are given in classes from 2 cm to greater than 5 cm in 1 cm increments, with the lower end being inclusive and the upper end exclusive: the grid value 2 represents values from 2 cm (incl.) to 3 cm (excl.).

Created by MeteoSwiss within the scope of the project Hail Climate Switzerland.