Is it raining harder and harder? Yes, it’s not just your imagination

During the past century, intense daily rainfall events have become about 26% more frequent and 12% stronger in Switzerland. Keystone / Gian Ehrenzeller

Rain may be falling less often, but when it does, it is increasingly intense. In Switzerland, that is not just a perception. Observations show that heavy rainfall has become stronger in recent decades, and climate projections suggest the trend will continue, with short-duration downpours becoming up to 30% more intense in the future.

Historical measurements from Switzerland’s rain-gauge network reveal how precipitation patterns in Switzerland have changed.

“Since the 1980s, when instruments capable of recording rainfall at ten-minute intervals came into use, the intensity of heavy rainfall over ten-minute periods has increased by around 20%,” says meteorologist Luca Nisi.

Over a longer timeframe the trend is equally clear. During the past century, intense daily rainfall events have become about 26% more frequent and roughly 12% stronger. The driving force behind this trend, which is increasing our vulnerability to extreme events, is the familiar culprit: global warming.

Warmer atmosphere holds more water

To understand why climate change affects rainfall, we need to look at a basic law of atmospheric physics. “If the temperature rises by 1°C, the air can hold around 7% more water vapour,” says Nisi.

The principle is described by the Clausius–Clapeyron equation. As temperatures rise, evaporation from soils, lakes and oceans increases, allowing more water vapour to accumulate in the atmosphere.

When the conditions for condensation and precipitation are met, that additional moisture can translate into heavier rainfall.

Drier summers, heavier downpours in future

Here lies one of climate change’s most striking paradoxes. According to the CH2025 climate scenariosExternal link, total annual precipitation in Switzerland is not expected to change dramatically. But its seasonal distribution will change: summers will be drier on average, while winters are expected to become wetter.

Change in average precipitation and extreme precipitation in Switzerland. Swiss Federal Office of Meteorology and Climatology MeteoSwiss, ETH Zurich,

The strongest increase is forecast for short-lived, high intensity rainfall associated with thunderstorms. In a world that is 3°C warmer, hourly rainfall could increase by around 10% to 30%, while daily rainfall could rise by 5% to 25%.

“You can really see how the increase is linked above all to storm-related rainfall,” says Nisi. South of the Alps, where flooding is already a significant risk, this is particularly important. Many people immediately think of the devastating Vallemaggia floods in 2024. But Nisi adds: “This does not mean that every event will reach that level of intensity, but intense episodes will become more frequent than in the past.”

The Alps and southern Switzerland have always been vulnerable to heavy rainfall because of the presence of the mountains and proximity to the Mediterranean. Climate change is now amplifying those natural conditions.

Return periods cut in half

A rainfall event that, over the 1991–2020 reference period, occurred on average once every 50 years could, in a world that is 3°C warmer, occur on average every 25 years.

Those events are also likely to be stronger. “We expect around 11% more rainfall,” says Nisi. This means that an extreme event that today produces 200 millimetres of rain in 24 hours could produce over 220 millimetres under that scenario.

Trends in daily heavy rainfall over the period 1901–2024, as observed at Swiss measuring stations. (Bauer and Scherrer (2024), updated and adapted) MeteoSwiss

When drought fuels flooding

Another apparent contradiction emerges: how can drier summers lead to greater flood risk?

“As we have seen this summer of 2026, severely parched soil absorbs water less effectively during the early stages of a very intense storm,” says Nisi.

When the ground cannot absorb rainfall fast enough, water runs across the surface instead, rapidly reaching rivers, streams and low-lying areas. This can worsen flooding, debris flows and landslides.

“Surface runoff is what actually causes a great deal of damage,” the meteorologist notes.

What about winter?

The changes are not confined to the warm season. As temperatures rise, the snow line continues to move higher, and an increasing proportion of precipitation will fall as rain at lower altitudes.

“It is reasonable to assume that, with increased winter rainfall and a general rise in the frequency of heavy downpours, the flood season will extend further into the year than in the past,” explains Nisi.

For Switzerland this poses a growing adaptation challenge. Roads, bridges, embankments, drainage networks and other infrastructure are typically designed using historical climate data. But Nisi warns: “If these volumes increase, it is important to adapt our infrastructure to avoid problems.”

Translated from Italian, sub-edited by Simon Bradley/ts