Understanding hail risks (2019)

Background
Extreme hailstorms are associated with very warm (convective) weather conditions and can cause a lot of damage in very little time. In fact, the largest ever insured disaster in the Netherlands is a recent hailstorm (July 2016 in the province of Brabant) which costed about €600 million, and in Germany five large hailstorm resulted in €2500 million damage in 2013.

Renewable energy is playing an even larger role to make our housing more sustainable, with solar panels on roofs being an important way to reduce the carbon footprint of homes. At the same time, large hail events are expected to occur more often in a warmer future, increasing the risk of damage to solar panels. Uncertainty in the probability of occurrence and higher levels of risk can lead to higher insurance premiums or even reduce the overall insurability, which could in turn inhibit entrepreneurs and citizens to invest in such renewable energy solutions. It is thus important to determine the actual probabilities associated with damaging hailstorms, and how solar panels are affected by such events.

What we did
In order to understand such hail risks better, we followed two paths of investigation: 

1. With the KNMI, we developed a database of hail events from radar data (2008-2019) and observations from written sources (1974-2019). With this database the probability of hail storms with particular magnitudes is determined and areas that are at high risk are identified.
2. Investigating the vulnerability of solar panels to hail stones based on damage records. Here we look at the relationship between hail stone size and damage to solar panels. This is combined with observations from satellite photos to investigate whether the angle and orientation of the solar panel have an influence on the damage.

What we found

• Annual hail risk (%) Most of the damaging hail events are reported in the summer months of May to August (~80%), and at the end of the afternoon (>50% occurs between 14.00-19.00).
• Observations from 1975-2019 show that an extreme hailstorm with hail stones of 7 cm or more occurs once in 10 years in the Netherlands. 
• The probability of hail is lowest in the North-West, and increases towards the South-East.
• Radar data from 2008-2019 shows that in the province Limburg the probability to be hit by damaging hail (hail stones >2 cm) is over 15% per year, as opposed to <5% per year in Noord-Holland and Friesland.
• Hailstorms typically affect areas of a couple of thousand km², with larger hail stones associated with larger storms.
• Damage to solar panels can be visible, but also invisible to the naked eye (so-called micro-cracks). Sometimes micro-cracks do not initially result in lower energy yield, but can grow over the course of months before resulting in a rapid decline of power output. 
• Damage to solar panels occurs at hail sizes of about 2 cm in diameter, with visible damage dominating from hail sizes of 3-4 cm.
• The orientation of solar panels also significantly influences damage, with lower damage when panels are not oriented in the direction the hailstorm is coming from.
• The angle of the panel also seems to have an effect, but less significant than the stone size and orientation.
• There is a need for regulation of standards and rules in the relatively new market of solar panels. This would also improve the insurability of solar panels, for which there is now no consensus between insurance companies within the Netherlands and in Europe.

Contact information: Dr Hans de Moel.

For more information see below the reports on hail database and the vulnerability of solar panels:
• Link rapport Hail Climatology: https://research.vu.nl/en/publications/a-hail-climatology-of-the-netherlands
• Link rapport Vulnerability Solar Panels: https://research.vu.nl/en/publications/the-vulnerability-of-solar-panels-to-hail