Hail Climate Switzerland

Every year, hail storms cause devastating damage. As small-scale weather extremes, they are difficult to measure, making hail a particularly challenging natural hazard. With a new generation of weather radar data and innovative reference measurements becoming available, it is now time to improve and standardize the basis for risk assessment.

Das Bild zeigt grosse Hagelkörner, die im Rasen liegen.

Hail damage to buildings, cars, and agricultural crops amount to millions of euros annually and so impressively demonstrate the effects of severe hail storms. This damage potential will most likely be of concern also in the future. One reason for this are steadily increasing assets and values as for example, insulation boards on houses which can be hit and damaged by hail. Another reason is that climate change could increase the risk even further, as higher temperatures and increased humidity provide important ingredients for more violent thunderstorms. However, there is still a lot of research to be done on how these developments will affect hail occurrence on the ground.

Scientific references to climate change and extreme weather

The most important basis for reliably estimating the current and future risk of hail are measurement methods with a high spatial and temporal resolution. Furthermore, these data need to be available over many years in order to evaluate it on climatological time scales.

The priority theme «Hail Climate Switzerland» develops and improves long-term observational hail data and produces products that can be used across public and private sectors. Within the project «Hail Climate Switzerland 175 (duration: 2018-2021), several partners currently develop a new climatological basis for assessing hail risk. The project is run as a public-private partnership led by the Federal Office of Meteorology and Climatology MeteoSwiss.

To provide this climate service, the project brings together the knowledge and needs of several partner organizations from the insurance, construction, civil protection, and agriculture sectors. This collaboration ensures provision of user-friendly, application-oriented climate information.

Goals

In the project, a climatology of the frequency and intensity of hail storms in Switzerland will be developed and established as a national reference. It will provide several products of hail information for various user groups.

Products (Available from 2021)

Maps and data on the frequency of hail days in Switzerland

The frequency maps show the average annual number of expected hail days in Switzerland based on radar measurements of the last twenty years. The underlying annual and monthly analyses of the observation data are also made available. The query can be made for the variables «Probability of Hail» (POH) and «Maximum Expected Severe Hail Size» (MESHS) for different threshold values. This is necessary because, for example, smaller hailstones can already cause significant losses in the agricultural sector, while damage to cars or roofs is only expected from hail storms with larger stone sizes.

Time series and further statistical assessments of the inter-annual variability of hail days in Switzerland will also be included in the product range.

All hail frequency information will be freely available as graphics and georeferenced raster data once the project is completed.

In the Jura, the Mittelland and the Alps, there are on average one to two days between April and September on which the radar-derived hail probability is at least 80 percent. In the Alps, the hail probability is lower with an average of less than one day per season, while on the southern side of the Alps it is sometimes more than two days with a radar-derived hail probability of at least 80 percent. — A preliminary evaluation of the average annual number of days on which the radar-derived probability of hail («Probabiliy of Hail», POH for short) is at least 80 %. The hail season is considered to be the period between 1 April and 30 September. The white crosses show the locations of the five MeteoSwiss weather radars. Source: MeteoSwiss.

Maps and data of the estimated return periods of hailstone sizes

These maps show which hailstone sizes (e.g. 3 cm) are expected at least once in a given period, e.g. fifty years. This period is called the return period of the hailstone size. In contrast to the frequency maps, which are based exclusively on observations, the relatively short data series must be extrapolated in order to calculate return periods. For this purpose, hail observations are combined with other climate variables and manifold using advanced statistical methods. Information on the uncertainties, which are an unavoidable and important concomitant feature of these analyses, are also part of this type of climate information.

The frequency information will be made freely available as graphics and georeferenced raster data at the end of the project.

What does the return period mean?
A well-known return period is 100 years. However, a «100-year event» is often misinterpreted as one that occurs (only) «once every 100 years». If the hailstone size determined for a 100-year event at a location is 4 cm, the estimated probability of observing a hailstone of 4 cm in size at this location is 1 % each year. The probability that such a hailstone will occur at least once in a period of 30 years is 26 %! This is calculated according to the principles of probability theory: In each year there is a 99 % probability that no 100-year event will occur (1-1/100 = 0.99), the probability that no 100-year event will occur over 30 years is 0.9930, i.e. 74 %. The probability that such a large hailstorm will occur at least once in this period is the complementary probability, i.e. 26 %.

According to a preliminary MeteoSwiss evaluation, the hailstone sizes to be expected for a 2-year event are less than 20 mm in most regions. They lie between 20 and 40 mm along the foothills of the Pre-Alps. — Maps of the return periods are created for different return intervals (e.g. 2 or 50 years) and show how frequently certain hailstone sizes are expected to occur at different locations. Source: MeteoSwiss.

Catalogue of hail events (by the day)

The event catalogue compiles maps and data of individual hailstorms in Switzerland as derived from radar data. For each event, the maximum hail probabilities for a given day, so-called footprints of a hail event, are provided. The cell track and the area affected by individual hail cells become visible here.

The individual event information will be available at the end of the project for a fee in accordance with the Act on Meteorology and Climatology (MetV).

The map shows a possible footprint of a Swiss hail day. Five hail cells are visible: two to the west, one south-east of Lake Geneva, one east of Bern and the last reaching into the Alpine region from the southern side of the Alps. The maximum hailstone size from radar is 20 mm for all hail cells. — Information on individual hailstorms is particularly important in post-event management. The individual hail cells become visible in so-called footprints of a hail day. The white crosses show the locations of the five MeteoSwiss weather radars. Source: MeteoSwiss.

The aims of the «Hail Climate Switzerland» project can be divided into two sub-goals:

To prepare operational, long-term, and quality-tested hail data from Swiss weather radars
To climatologically analyze these data and model the probabilities of occurrence of (rare) hailstorms and the expected grain sizes

A third, cross-project objective is to promote cooperation between the public and private sectors in order to develop applicable products and climate services together with users.

In greater detail

Highlights

Public-private partnership project in which the regular and intensive exchange between the partner organizations from various affected sectors supports the application-oriented development of climate services.
Hail blog series in MeteoSwiss Blog (german; Teil 1, Teil 2, Teil 3, Teil 4, Teil 5)

Partner organizations

All project partners delegate expert representatives who closely accompany the project. Project progress, expectations, and methods are regularly discussed in exchange forums. All partner organizations bring their expertise and many years of experience in dealing with the natural hazard of hail to the project.

Project sponsors are:

The Mobiliar Lab for Natural Risks at the Oeschger Centre for Climate Change Research at the University of Bern is an associated partner with scientific expertise and advises the project scientifically.