Thermal Trace is a new interactive web-based tool, launched in August 2025, for monitoring heat and cold stress across the globe and how it has changed over time - from 1940 to five days behind real time: thermaltrace.climate.copernicus.eu
Thermal stress refers to the negative health impacts caused by exposure to extreme thermal conditions, for example during heatwaves and cold spells. This new app offers a practical tool for both climate monitoring and understanding the impact of weather and climate on health.
Users can use Thermal Trace to explore questions such as:
- Which locations have recently experienced extreme heat stress?
- What was the highest feels-like temperature during the heatwaves this summer?
- How many days of heat stress occurred in the month I was born?
- Over the last decade, how many tropical nights has my local area experienced each summer?
- How did last winter's cold stress compare to the long-term average?
The data behind Thermal Trace
The dataset used is the ERA5 reanalysis, primarily the ERA5-HEAT Universal Thermal Climate Index (UTCI) (https://cds.climate.copernicus.eu/datasets/derived-utci-historical). This represents a feels-like temperature, accounting for temperature, humidity, wind speed, solar radiation and how the human body responds to the thermal environment.
Thermal Trace provides intuitive information about heat and cold stress through interactive global maps and location-based charts. Users can explore daily, monthly, seasonal, and annual maximum and minimum feels-like temperatures, the peak heat or cold stress category experienced, or the number of heat and cold stress days or tropical nights over a given period, as well as changes over time. The tool can also display these as anomalies relative to the average for the 1991–2020 reference period.
These statistics were also published in the Copernicus Climate Data Store (CDS) as part of the launch of Thermal Trace.
Powerful features and innovative design
Thermal Trace takes advantage of Zarr, a modern file format that makes it easy to store and quickly access large datasets over the cloud. This enables features such as real-time calculation of climatologies and extremes, and the creation of bespoke maps based on user input.
With most Internet traffic now on mobile devices, Thermal Trace has been designed to work seamlessly on both mobile and desktop. The visual style follows modern design trends, such as a glass-like minimal user interface for a professional look. To help new users unlock the app's full potential, interactive tutorials with gamified elements make the learning process engaging and encourage exploration into the deeper functionality.
Users can search for a specific location or click or tap on the map to choose different time periods. The charts can be downloaded as images and the plotted data can be downloaded as csv files. In a coming update, Python notebooks will be added to further describe and demonstrate the data processing for maximal transparency. Comprehensive FAQs provide clear and accessible information.
Investigating summer 2025 in Europe using Thermal Trace
Summer 2025 saw heatwave conditions affecting many areas of Europe, and the maximum feels-like temperatures (UTCI) for the summer were generally above average, with northeastern Spain, southwestern France and much of Fennoscandia seeing some of the largest anomalies. Some of the highest numbers of heat stress days occurred in southern Spain and western Türkiye, with up to around 80 days of at least 'very strong heat stress' and a week of 'extreme heat stress'.
AEMET reported that Spain experienced its most intense heatwave on record in August, lasting 16 days. Thermal Trace indicates that feels-like temperatures reached up to 49°C in southern parts of the country, around 11°C above average. The threshold for 'extreme heat stress' – at which point it is imperative to take actions to avoid heat stroke - is 46°C. This heatwave also impacted Portugal and France. More information can be found in the C3S monthly climate bulletin for August 2025 (https://climate.copernicus.eu/climate-bulletins).
By presenting decades' worth of data on thermal stress in an accessible way, Thermal Trace helps connect weather and climate with impacts on health, offering a new resource for research, planning, and awareness.
The content, design, implementation, technical infrastructure and dissemination of Thermal Trace result from a collaborative effort by colleagues across ECMWF, as entrusted entity for the Copernicus Climate Change Service (C3S), and were additionally supported by the Horizon Europe ASPECT project.