10 questions on heavy rainfall for Prof. Axel Bronstert (hydrologist at the University of Potsdam) & Prof. Jan Härter (physicist at the University of Potsdam)

1. When is rain considered heavy rain?
   Heavy rain refers to large amounts of precipitation per unit of time. It usually falls from convective clouds (thunderclouds). Heavy rain is usually localised (covering an area of approx. 10–100 km²) and short-lived (up to a maximum of 6 hours) and can lead to rapidly rising water levels and flooding. Heavy rain often causes soil erosion. The German Weather Service issues heavy rain warnings in three levels (when the following thresholds are expected to be exceeded):
   • Rainfall of 15 to 25 l/m² in 1 hour or 20 to 35 l/m² in 6 hours (significant weather warning)
   • Rainfall > 25 to 40 l/m² in 1 hour or > 35 l/m² to 60 l/m² in 6 hours (severe weather warning)
   • Rainfall > 40 l/m² in 1 hour or > 60 l/m² in 6 hours (extreme weather warning)
2. How often do such heavy rains occur? Are they related to the season?
   Heavy rains occur rarely. For a specific location, this can be significantly less than once a year. The greater the intensity of a heavy rain (see warning levels), the less frequently it occurs. Heavy rains occur more often in the warm seasons. This is because warm air can hold more water vapour. The rain that forms from the condensed water vapour in warm air is therefore heavier than rain that forms from colder air. 
    
3. What changes in heavy rainfall do we expect as a result of climate change? And why?
   As the climate gets warmer, there are more warm days, which increases the likelihood and frequency of heat storms. We have already demonstrated the more frequent and intense heavy rainfall in a warmer climate based on long-term rainfall data.
    
4. How is climate change affecting annual precipitation in Germany?
   No statistically significant trends in total annual precipitation have been identified for any region of Germany to date. However, it is clear that there is less rain overall in the summer months and more in the winter months. That said, there are always periods with drier and wetter years. For example, the 2010s were drier than the long-term average, while 2023 and 2024 were wetter.
    
5. Is heavy rainfall equally problematic in all regions of Germany?
   In principle, heavy rainfall can occur anywhere in Germany. However, it can sometimes be even heavier at the edges of mountain ranges than in low-lying areas due to the clouds rising on the mountain slopes. The situation is different when it comes to flooding caused by heavy rainfall, known as flash floods. These are particularly pronounced in areas where the ground can only absorb a small amount of rainwater (e.g. in sealed areas or areas with low permeability and/or flat ground) and where surface runoff can flow together very quickly due to the topography (mountains) or urban ‘runoff paths’ (roads or drainage ditches).  
     
6. What is the situation in Germany with regard to protection against extreme precipitation?
   Due to the aforementioned effects of global warming, heavy rainfall is occurring more frequently and/or with greater intensity. This is also leading to an increase in local flash floods. However, the way in which such events are dealt with has also improved. For example, ‘heavy rainfall hazard maps’ are now published for almost all regions of Germany, showing areas that are particularly vulnerable to flooding and where special protective measures can be taken, such as structural measures or reducing the potential damage in these regions. Meteorological and hydrological forecasting of such events has also been improved.
    
7. What role do so-called sponge cities play in dealing with extreme precipitation?
   The principle of the sponge city aims to reduce the runoff that forms during heavy rainfall by retaining a larger proportion of the rainwater. This can certainly lead to a reduction in flooding. However, the more intense the heavy rainfall, the smaller (relatively speaking) this increased retention effect is.
    
8. Are there already good examples of cities that have increased their water storage capacity or otherwise cope well with heavy rainfall?
   Many cities in Germany, including Berlin, are attempting to reduce the potential for flooding in populated areas by implementing measures based on the ‘sponge city’ principle. However, this can only be achieved through long-term measures (lasting decades) and is also costly.
    
9. Sometimes you see the term ‘heavy rain dementia’ in the media. What does it mean?
   If it has been a relatively long time (at least more than five years), the population of a city has often forgotten the possibility and potential damage of heavy rain. As a result, little attention is paid to prevention and reducing the potential for damage. This is also referred to as ‘heavy rain dementia’.
    
10. What should each of us pay attention to in connection with possible heavy rainfall?
    Do not risk human lives, i.e. stay away from flood areas. Do not try to “rescue” cars, as floating cars are traps for their drivers, sometimes even deadly ones. If houses are located in potential flood areas, do not store valuable items in basements. Bridges and waterways can become blocked (“logged”) and cause very rapid and severe local flooding.