Weather stations measure record-breaking temperatures in Freiburg City
Freiburg, 29/06/2026
The current heatwave has already seen temperatures in Freiburg rise higher than during the so-called ‘summer of the century’ in 2003, even as early as the end of June. Freiburg city centre has recently experienced thirteen tropical nights in a row – more than in August 2003. Prof. Dr Andreas Christen, a meteorologist at the University of Freiburg, has analysed the latest measurement data and explains what they mean for climate-adapted urban planning.
Mr Christen, The current heatwave has led to a particularly high number of tropical nights in Freiburg. How would you interpret this trend from a climatological perspective?
The recent heatwave has been exceptional, particularly in terms of its duration, intensity and timing. We have been taking continuous readings since 2000 at the University of Freiburg’s weather station, which is situated 50 metres above ground level on top of one of the University’s research buildings. On 27 June 2026, we recorded a maximum temperature of 38.8 degrees Celsius there over a period of ten minutes. This is warmer than the highest air temperature we recorded over a ten-minute period in 2003 – and is therefore our all-time temperature record. Back then, it stood at 38.0 degrees. The night leading into Sunday, with a minimum temperature of 26.1 degrees, was the warmest night since we began taking measurements. During this heatwave, there were also thirteen consecutive tropical nights up to 29 June. Over the last ten years, there have been a maximum of five per year. By way of comparison: during the so-called ‘summer of the century’ in 2003, there were twelve consecutive tropical nights. This means that, as early as June this year, we have already had more tropical nights than in August 2003.
You are recording temperatures in Freiburg with high spatial resolution and have observed, in some cases, significant differences between individual neighbourhoods. Why is that?
These differences are primarily due to building density, land sealing, the proportion of green space and air circulation. Since 2022, we have been recording temperatures across the Freiburg urban area using 42 ground-level weather stations. These stations take readings at a height of three metres in urban canyons, parks or above agricultural land, thereby reflecting the air temperature to which people are actually exposed in their daily lives. On 27 June, the 40-degree mark was exceeded at 29 measuring stations. The highest reading was recorded in front of the Medical Center – University of Freiburg at 43.1 degrees. Whilst these measurements are not directly comparable with the standard measurements taken by the German Weather Service, which are recorded outside densely developled areas over short grass, they do provide a very good indication of the actual air temperature in the city.
The graph shows the maximum air temperatures on June 27, 2026—the hottest day of the heat wave—in Freiburg and the surrounding area. The data comes from the University of Freiburg’s street-level weather stations and is based on one-minute measurements.
The differences become even more pronounced at night. In the city centre, as many as 13 tropical nights were recorded at ground level up to 29 June – whereas in the western suburbs, the figure ranged from 0 to 5. This is due to the urban heat island effect: buildings and sealed surfaces store heat during the day, which they then release at night. The denser and more sealed the city, the warmer it is there at night. Added to this is the fact that night-time winds cool some parts of the city.
The graph shows the nighttime minimum temperatures during the night of June 27–28, 2026—the warmest night of the heat wave so far—in Freiburg and the surrounding area. The data comes from the University of Freiburg’s street-level weather stations and is based on one-minute measurements. Graph: University of Freiburg
From a meteorological perspective, which measures are particularly effective in making cities more resilient to rising temperatures?
We can mitigate the effects of heat in cities, primarily through measures that reduce solar radiation and heat retention and promote ventilation. These include shade provided by urban trees, light-coloured rooftops, green facades, and the reduction of waste heat from engines and machinery. It is also important to keep ‘cool air corridors’ clear. These are areas within a city where cooler air from the surrounding countryside can flow in at night and help to cool the city.
How, specifically, can your research findings be incorporated into urban planning and decision-making at the municipal level?
Our measurements are primarily used in research and in modelling adaptation measures. They form an important basis for testing weather models for Freiburg. On this basis, we can then simulate the effect of various measures to combat the heat. For example, we carried out a computer simulation of the influence of trees on heat stress in the Wiehre district of Freiburg. The results show that, with the current tree population, heat stress amounts to 53 hours per year, whereas without trees it would be 98 hours per year. The existing trees therefore reduce heat stress by almost half. This demonstrates how effective the preservation of large urban trees is, even on private land. Planting additional trees is also beneficial, but their effects are only felt in the long term, once they provide sufficient shade.
In your view, what is the most important task facing cities such as Freiburg if they are to remain liveable in future?
The increase in the frequency and intensity of heatwaves is directly linked to global greenhouse gas emissions. Global climate protection through emissions reduction therefore remains the most effective measure for mitigating this global trend. At the same time, we need consistent local adaptations: climate-adapted urban planning incorporating shade and water features, as well as measures that allow the city to cool down again at night. A nuanced approach is also required when it comes to the often-controversial topic of air conditioning. In critical infrastructure such as care homes, hospitals, schools and nurseries, the issue is not one of comfort, but of health and, in extreme cases, survival. Air conditioning need not be harmful to the climate. This is because, during the hottest part of the day, there is sufficient energy from photovoltaic systems to cool at least critical infrastructure. Equally important is the protection of particularly vulnerable groups – such as young children, the elderly and people with reduced mobility. Heat is therefore also a matter of climate justice: those living in small, extremely hot flats are significantly more affected than people with more options for seeking relief.
About
Prof. Dr Andreas Christen heads the Chair of Environmental Meteorology at the Faculty of Environment and Natural Resources at the University of Freiburg. He is the Principal Investigator of the Integrated Carbon Observation System (ICOS) site in Hartheim am Rhein, as well as numerous other research projects and consortia, such as the ERC Synergy Grant ‘urbisphere’, the EU project ‘UrbanAIR’ and the Cluster of Excellence ‘Future Forests’.