Cities experience higher temperatures than their rural surroundings. These higher temperatures can negatively impact human health, including causing cardiovascular and respiratory disorders, as well as heat stroke. It is therefore crucial to lower heat stress in urban environments.
Cities are ‘Heat Islands’
An ‘urban heat island (UHI)’ is an urban area that is significantly warmer than its rural surroundings due to artificial infrastructure and human activities. Whilst urban areas have a higher proportion of paved ground, rural areas are covered in grass, crops, shrubs or forest. This vegetation helps to cool the air whereas asphalt and concrete absorb heat, causing temperatures to rise. Furthermore, buildings and narrow streets trap heat by reducing air flow. Human activities such as warming buildings and driving cars also add heat to surroundings.
All these factors contribute to the urban heat island effect, which is most pronounced during the night, when temperatures in urban areas can be up to 10°C higher than in rural areas. This is due to retained heat in structures such as buildings and roads being released during the night.
Larger cities tend to experience stronger heat stress; the centres of London and Paris, for example, regularly record temperatures of around 4°C higher than rural surroundings at night. Both of these cities, and many others around Europe and the world, experience heat stress that leads to public health issues.
Urban planners and local governments want to design strategies to reduce this heat stress. These strategies could involve increasing so-called green-blue infrastructure, including enhanced vegetation cover (for example through green roofs) and more water surface. But to make these changes, they need to know how temperature varies spatially within the urban environment that they are responsible for.
To learn more about heat stress in cities, see the article we published about this earlier.
Measuring City ‘Hotspots’ from Space
The Copernicus Climate Change Service (C3S) offers specific satellite data products to map heat islands in cities. The spatial variation of temperature can be obtained using an urban climate model. Using C3S ERA5 climate reanalysis data on air temperature, specific humidity, relative humidity and wind speed, VITO applied the UrbClim® model to provide urban climate information for 100 European cities at a high spatial resolution of 100 metres. UrbClim® has been successfully used and validated for many cities worldwide.
The example of Vienna
Between 1961 and 1990, Vienna experienced on average 9.6 heat days per year. From 1981 to 2010, the number of heat days rose to 15. According to forecasts, there will be an average of 19 hot days between 2021 and 2050, and 41 between 2071 and 2100. Moreover, the population is expected to increase from 1.8 million to 2 million by 2029. To protect the population from the consequences of heat events, the Energy Planning department of Vienna decided to take action.
When city planners and Earth Observation specialists of urban heat consultancy firm Ecoten looked at existing heat island maps of the city, they quickly realised that these heat maps only reveal part of the problem. The fact that an area in the city is warmer than other areas doesn’t always mean that people are suffering from it. Some areas are non-residential for example, or benefit from cooling factors, like green areas or bodies of water.
Urban Heat Vulnerability Index (UHVI)
Ecoten developed a heat stress indicator that considers several factors of neighbourhoods within cities, to provide a detailed heat vulnerability score of these individual neighbourhoods:
- Exposure: Measured by studying and quantifying the occurrence of high temperatures within the city over a number of years, using Earth Observation data, for example provided by Copernicus C3S.
- Sensitivity: This index represents the percentage of highly vulnerable population. This can be measured as population density in general, or zooming in on the spread of vulnerable groups within the population that are extra sensitive to heat, like young children and the elderly.
- Adaptive capacity: Measured by identifying greeneries and water-bodies. It represents the ability of the ecosystem to cope with heat events.
Combining these three indices leads to the generation of a detailed heat vulnerability map of the city, in this example Vienna:
Without any action, the most vulnerable areas in the city will get worse over time. The top 10 districts ranked by their average Urban Heat Vulnerability Index present a very little vegetation. The map also shows that even in less urbanized areas, heat vulnerability points are likely to appear. Through the study, Ecoten found that by 2050, Vienna will heat up to 8 degrees if no action is taken.
What can cities do to reduce heat stress?
Ecoten identified several solutions for city planners to reduce heat vulnerability:
- Vegetalise: Implementation of vegetation (trees, grass, green roofs and walls) creating protective shadows and evapotranspiration.
- Humidify: Implementation of water bodies within urban areas, acting like a heat captor to cool the city and improve the quality of life.
- Brighten up: Implementation of different materials, colours, as well as shapes of structures and buildings to uptake heat from solar radiations. Remember those white-washed houses in the Mediterranean? White paint helps!
The Cool Streets project
Based on these findings, the city of Vienna is now developing “Cool Streets“: places for people to stay outdoors, play games, and socialise. This program intends to provide additional seating, cooling systems such as mist showers and establish parking ban to allow citizens enjoy the city at its best. After interviewing 518 citizens results showed that „Cool Streets“ was a great success and should be continued next year. The municipality plans to extend this measure to all 23 Viennese districts.
Demonstrating heat stress in European cities, Copernicus C3S blog
Urban Heat Vulnerability Assessment of Vienna, Austria, Ecoten website news section