With the transition to shorter and cooler days, we have the opportunity to look back on one of the hottest days of this summer and reflect on the insights gained about heat stress in everyday environments.

On July 2, 2025, the Bochum Urban Climate Lab (BUCL) conducted climate measurements at a Kindergarten in Bochum as part of the WATCH (“Water for Children in Climate Change”) research project. On that day, the highest air temperature of the year was measured at 37.4 °C. Between 08:00 and 10:00 UTC, the air temperature and the Universal Thermal Climate Index (UTCI) were recorded at two measuring points to investigate the effects of extreme heat on fluid loss in children.

The measurement results show a significant increase in thermal stress: around 30 minutes before the observations began, the UTCI values already exceeded the threshold of 32 °C, which corresponds to severe heat stress. At around 09:45 UTC, the threshold for very severe heat stress was exceeded at 38 °C. At the same time, air temperatures also continued to rise, highlighting the pronounced morning heat stress at the measurement site.

Accompanying physiological data showed that the children studied lost almost twice as much fluid as on a thermally neutral comparison day. This value is already in the range where the first symptoms of dehydration can occur.

These results highlight the urgency of providing targeted support to childcare facilities to help them adapt their daily routines, drinking habits, and outdoor activities to rising urban temperatures. The ongoing work of the BUCL as part of the WATCH project is making an important contribution to the development of evidence-based recommendations for healthier and safer childcare environments under changing climatic conditions.