In response to the increasing frequency and intensity of heat waves due to climate change, Copernicus Climate Change Service (C3S) and ECMWF (European Centre for Medium-Range Weather Forecasts) have developed an innovative application called Thermal Trace. This new tool provides a fast, flexible, and free way to explore, visualize, share, and extract more than 80 years of heat stress data, with an emphasis on monitoring the impact of heat on health.

What does Thermal Trace offer?

The app allows users to visualize changes in heat stress days or tropical nights over time, as well as understand and monitor extreme values of heat and cold stress in our warming world. Thermal Trace provides information from 1940 to five days before present. With an intuitive interface, users can view different variables and time ranges, easily visualize data through maps and graphs. The application also allows downloading data from graphs, and the full dataset is available in the Climate Data Store.

Why is This Important?

Heat stress is a critical issue that can have fatal consequences. According to the WHO, heat stress is one of the leading causes of weather-related deaths. The European State of the Climate 2024 report states that in 2023, approximately 48,000 heat-related deathsThermal Trace and its associated dataset are a valuable resource for the general public, policymakers, journalists, scientists, and decision-makers from various industries to better understand, monitor the past, and prepare for the future.

Universal Thermal Climate Index (UTCI)

The core of the data in Thermal Trace is Universal Thermal Climate Index (UTCI)UTCI is important because it takes into account not only temperature, but also other variables such as humidity, wind speed, solar radiation and heat radiated by the environment, and how the human body responds to them. UTCI provides a “feels like temperature” in °C and categorizes the level of heat stress on the body:

• Heat stress:
  • Moderate: 26°C to 32°C
  • Strong: 32°C to 38°C
  • Very strong: 38°C to 46°C
  • Extreme: above 46°C
• Cold stress:
  • Mild: 9°C to 0°C
  • Medium: 0°C to -13°C
  • Strong: -13°C to -27°C
  • Very strong: -27°C to -40°C
  • Extreme: below -40°C

Navigation and application features

The app is easy to navigate and offers access to a wealth of information. Users can select variables, time frame (daily, monthly, seasonal and annual) and location of interest (using a map, search, geolocation or defining a small area). The app allows:

• Browse global maps peak cold or heat stress.
• Go back in time and view cold and heat stress data for specific events, such as The "Great Winter" of 1962/63 in Europe or heat waves in 2025.
• Obtain the number of days of heat or cold stress and compare the given period with the climatological average for the years 1991-2020.
• Create heat maps and visualizations similar to the well-known “climate bars” that show, for example, the number of days with severe heat stress per year.

Technological basics

Thermal Trace uses Zarr, an innovative open-source archive technology that allows applications to quickly process data on the fly. This format enables efficient processing of large multidimensional data sets, such as ERA5, by storing data in separately accessible chunks, ensuring faster retrieval and visualization.

It is important to note that while Thermal Trace allows you to examine recent events and changes over time, does not offer real-time event monitoring or predictionsYou should consult your local and national weather services for real-time information, forecasts, and advice on how to protect yourself from heat and cold stress during ongoing weather events.

The Thermal Trace application is part of Copernicus, the European Union's Earth monitoring program, which aims to benefit all European citizens. JRi