New GATE Institute study shows a significant increase in heat stress in Bulgarian cities, with evening temperatures up to 9.5°C higher during the compared heat periods in 2006 and 2025
The first national attribution study conducted by researchers at the GATE Institute and focused on heat stress in Bulgaria and Bulgarian cities shows a substantial increase in the risk of extreme heat. The analysis looks beyond air temperature alone, combining temperature, humidity, wind, and solar radiation to assess the actual heat load on the human body.
The study analyzes conditions at the national level and in 10 Bulgarian cities – Sofia, Plovdiv, Pleven, Blagoevgrad, Ruse, Vidin, Kardzhali, Sliven, Varna, and Burgas. The cities were selected to represent different parts of the country, terrain and climatic conditions, proximity to bodies of water, and different characteristics of the urban environment.
Key findings
Heat stress reaches extreme levels. During the heat period studied in 2025, the WBGT index averaged around 32°C across the country – 2°C higher than in 2006, placing it in the “extreme heat stress” category. The UTCI reached 50°C – an increase of nearly 9°C.
The strongest change is observed in the evening. Modelled evening temperatures in 2025 were significantly higher than during the comparison period in 2006. The difference reached +9.5°C in Pleven, +8.5°C in Sofia, and +7.5°C in Plovdiv.
Evening cooling is also slowing down. In 2025, even at 8:00 p.m., the average Mean Radiant Temperature (MRT) reached around 45°C – nearly 6°C higher than in 2006. This shows that heated buildings, pavements, and other surfaces continue to release heat even after the hottest part of the day has ended.
The location of a city matters. Cities in the interior of the country show greater differences, while the change is significantly smaller in Varna and Burgas. Maritime influence, local weather conditions, and characteristics of the urban environment are among the factors determining heat stress.
What does this mean for people?
The temperature we see in the weather forecast does not, by itself, show how heat affects the human body. 35°C does not always feel like 35°C. Humidity, wind, direct solar radiation, and heat radiated by buildings and pavements can significantly increase heat stress.
That is why the study uses internationally recognized indices such as WBGT, PET, and UTCI, which combine these factors. WBGT is used to assess risk during outdoor work and sports, PET shows how a person perceives the thermal environment, while UTCI assesses the level of heat stress and the associated health risk.
The results show that heat stress is already reaching extreme categories, making adaptation to heat an increasingly important part of health protection and sustainable urban development.
What can municipalities and institutions do?
The results show that one risk does not mean one solution for all cities. Local data and assessments are needed so that measures can be targeted at the places and people at highest risk.
Recommended measures include heatwave action plans and early warning systems, the creation of cooling centers, cooling systems using dry mist and drinking water stations, increased shading and urban greenery, as well as adapting public buildings through cooling, good ventilation, and external shading.
Maps of areas with high heat risk are also needed, along with adjustments to working hours and outdoor activities during the hottest hours and specific measures for vulnerable groups. Coordination between municipalities, health and social services, employers, and non-governmental organizations is also important.
Local differences are key. Therefore, adaptation to extreme heat must take into account the local climate, urban structure, green spaces, and the way urban areas are used.
The study was led by Dr. Eng. Lidiya Vitanova, a researcher at the GATE Institute and a specialist in urban climate, heat stress, climate modelling, and urban planning.
WBGT – Wet Bulb Globe Temperature – Used primarily to assess heat stress during work and sports.
PET – Physiologically Equivalent Temperature – Shows how the human body perceives the thermal environment.
UTCI – Universal Thermal Climate Index – Assesses thermal comfort, heat stress, and the associated risk.
MRT – Mean Radiant Temperature – Shows the thermal impact of the sun and heated surrounding surfaces on the human body.