What does a heatwave sound like? What happens when we try to listen to an extreme climate event instead of just measuring it?
CIMA Research Foundation transformed real temperature data recorded in Paris between 28 and 30 July 2024 into a sonification: a sequence of sounds in which the pitch increases as the temperature rises. A sensory experiment designed to make the intensity and duration of an urban heatwave perceptible through hearing.
Warning: the video contains sharp and sudden sounds. We recommend listening at a moderate volume.
But that sound, hypnotic and piercing, is only the beginning of a deeper story. Because behind every heatwave lies a scientifically complex and relevant question: what was the relationship between extreme heat and drought in 2024?
To fully understand what happens during a hot summer like that of 2024, it is important to remember that heatwaves and droughts are distinct yet often interconnected phenomena. Heatwaves are defined based on climatic thresholds exceeded over consecutive days, while droughts appear in various forms: meteorological, agricultural, hydrological, snow-related or socio-economic. Analysing them requires specific indicators — such as SPI, SPEI, SMA and CDI — that can detect anomalies in precipitation, temperature, soil moisture or vegetation. We explored these concepts here.
Summer 2024: An European case study
The data collected during the summer of 2024 reveal three distinct climatic dynamics: all marked by high temperatures but differing in terms of hydrological impact.
In Paris, the meteorological heatwave recorded from 28 to 30 July was clearly felt by the population and captured in its full intensity through the sonification produced by CIMA Research Foundation. However, the indicators used at the European level did not detect significant signs of agricultural or hydrological drought, highlighting the complexity of the relationship between public perception and scientific analysis.
Madrid experienced prolonged periods of intense heat, but initially moist soils likely mitigated impacts on water reserves. The SPI (Standardized Precipitation Index) and CDI (Combined Drought Indicator) do not show any critical signals, though the late-July SPEI1 (Standardized Precipitation-Evapotranspiration Index at a one-month timescale) suggests a more fragile water balance due to increased evapotranspiration.
Rome faced a completely different situation: a soil moisture anomaly was already evident in May, and summer only worsened it. CDI and SMA (Soil Moisture Anomaly) indicate progressive deterioration, exacerbated by persistent heat and a lack of rainfall. Here, the transition to agricultural drought was rapid, and the data suggest a situation close to warning thresholds.
“One of the keys to understanding vulnerability is speed,” explains Luca Trotter, researcher in the Drought Area at CIMA Research Foundation. “A system can withstand weeks of heat if it has accumulated water, but it can collapse in just a few days if it is already in deficit.”
Flash Drought: when drought accelerates
This is the case with flash droughts — rapid-onset droughts that develop over days or weeks. These events, observed in Southern Europe in 2024 as well, are often triggered by heatwaves capable of drying out the soil in a very short time. In such contexts, the speed of change becomes more relevant than the duration of the event: conditions can shift from moist to critically dry in just a few days.
A key element in identifying these phenomena is the use of short-timescale indicators. SPEI1, in particular, is highly sensitive to rapid changes and can capture drought acceleration. Between 20 and 30 July 2024, for example, significant variations in SPEI1 values were observed in Spain and Italy, indicating the sudden onset of drought conditions. But with differences: in Rome, the phenomenon had already begun in spring, while in Madrid the heat, though intense, had less impact thanks to soils still capable of retaining water.
“When evapotranspiration and vegetation stress reinforce each other, the moisture curve breaks. The system evaporates more than it receives and quickly empties. If you don’t look at the right data, the signal is lost,” explains Michel Isabellon, researcher in the Drought Area at CIMA Research Foundation.
Listening to learn how to discern
The sonification produced by CIMA Research Foundation was created precisely with this purpose: not to replace scientific analysis, but to make it perceptible. A sound alone cannot explain a heatwave, but it can raise questions. It is the beginning of a process of awareness.
“It is difficult to define what a heatwave or a drought really is. The threshold changes depending on scale, impact, perception. But if we start to hear the climate, perhaps we also begin to understand where and when to act,” concludes Massimo Milelli, Head of the Meteorology and Climate Area at CIMA Research Foundation.
The sound of heat, then, is also an invitation: to observe, to integrate, to distinguish.
Because behind every thermal anomaly, there may be a hidden water risk.
And behind every risk, a choice.