Heavy Rain Has Increased Significantly

A new study led by GeoSphere Austria confirms for the first time the assumption that heavy rainfall events in Austria have become more intense.

The amount of rainfall within a day has increased by 8 percent in extreme events over the past 40 years. Extreme events within an hour now bring 15 percent more rain than before. The increase in hourly rainfall is directly related to climate warming. For the increase in daily amounts, this connection is less clear, with weather patterns playing a more significant role.

The study was published this week in the renowned journal Nature.

Many events and investigations in recent years have suggested an increase in heavy rainfall and the associated flooding. However, reliable evaluations of extreme rainfall amounts over short periods, such as hourly rainfall amounts, have been hampered by measurement uncertainties and limitations in data availability.

A study by GeoSphere Austria, the Technical University of Vienna, the Federal Ministry of Agriculture, Forestry, Regions, and Water Management, and the University of Graz now shows robust measurement data confirming that short-term heavy rainfall events have significantly intensified in recent decades, corresponding with the increase in flood events.

The study, Increasing hourly heavy rainfall in Austria reflected in flood changes, was published this week in Nature (Link to article, available from March 12, 2025, 5 PM).

Data from two independent Austrian measurement networks (GeoSphere Austria and the Hydrographic Service) with a total of 883 stations, covering the period from 1900 to 2023, were examined. The results of both networks align.

For the evaluation of hourly rainfall amounts, data from 1950 onward was used, as no usable hourly data exists from before that time. ‘From 1950 to 1980, the hourly values of heavy rainfall showed no trend. Over the 40 years from 1980 to the present, heavy rainfall amounts increased by about 15 percent,’ says Klaus Haslinger of GeoSphere Austria, the study’s lead author. ‘More precisely, an average heavy rain event between 2003 and 2023 brought 15 percent more water than an average event between 1950 and 1970.‘

The results are similar for all regions of Austria and align with studies from other European countries, as well as from India, Australia, and the USA.

‘This shows that the increase in hourly heavy rainfall amounts is primarily due to climate warming,’ says climate researcher Haslinger. ‘Per degree of warming, air can hold about seven percent more water vapor, as defined by the Clausius-Clapeyron equation. The past 40 years have brought about two degrees of warming in Austria, which closely matches the 15 percent increase in heavy rainfall observed in station measurements.’

The analysis of extreme daily rainfall amounts presents a more nuanced picture:

From 1900 to 1960, daily heavy rainfall amounts remained relatively constant, declining somewhat in the 1970s and 1980s. From the 1990s to the 2010s, daily heavy rainfall amounts increased by 8 percent, remaining at that level with a slight decline over the past ten years.

A notable feature of daily rainfall amounts is the north-south difference in Austria. ‘Daily heavy rainfall amounts fluctuate significantly more on the northern side of the Alps than in the south,’ says climate researcher Klaus Haslinger of GeoSphere Austria. ‘Additional studies have shown a strong correlation between rainfall amounts and large-scale weather patterns. More than previously thought, daily rainfall amounts are influenced by weather conditions rather than warming.’ The occurrence of specific low-pressure systems plays a crucial role, such as ‘Vb weather patterns’ (responsible for heavy rain in much of Austria), as well as the persistence of weather systems in specific locations.

A key finding of the study is that extreme daily rainfall amounts are strongly influenced by the development of large-scale weather patterns, such as the frequency of low-pressure systems moving from the Mediterranean to Austria. Due to the Alpine barrier, this results in significant regional differences.

The extreme rainfall amounts within an hour, as seen primarily during severe thunderstorms, respond mainly to climate warming. This trend is similar across different regions of Austria and globally.

Another focus of the study was the change in flood events. This aspect was primarily analyzed by the team at the Technical University of Vienna. In summary: Changes in daily heavy rainfall amounts correspond well with changes in large-scale flood events. The increase in hourly heavy rainfall amounts corresponds with an increase in localized flood events (+24 percent over the past 40 years).

The data and findings from this study are directly incorporated into the recently launched research project Water in Climate Change – Our Water Management 2050+, a collaboration between TU Vienna, the Federal Ministry of Agriculture, Forestry, Regions, and Water Management (BML), GeoSphere Austria, and the University of Graz.