Measuring heat stress in the city from space

 Measuring heat stress in the city from space

One of the most common terms we hear during the summer when discussing the weather in the city is the Heat Index. While the Heat Index can give you an idea of how dangerous the heat can be, there is another lesser known variable that may give a complete description, known as the Wet Bulb Globe Temperature (WBGT).

The Heat Index is a measure of how hot it feels to the human body, taking into account the temperature and the relative humidity. The higher either one value is, the higher the heat index and the greater the stress exerted on the body. Heat Index utilizes the temperature measured in the shade and therefore will likely be greater in the sun, where many activities take place. For temperatures taken in the full sunlight, Heat Indices can increase as much as 8°C. Humidity plays a major effect on cooling the body. If the humidity is high, sweat on the surface of the skin does not evaporate as quickly, slowing the cooling of the body. If the humidity is low, sweat evaporates too fast, leading to dehydration.

The left image shows the air temperature for Madrid on 25 June 2008 at 22:18 UTC. The difference between the minimum and maximum temperatures in the colour legend is 6°C. The right image shows the sealed soil surfaces percentage (provided by the European Environmental Agency). The position of the Retiro Park is at the centre of the small rectangle, and shows an almost non-existent heat island at this time of the day. This demonstrates the important role that parks play in cooling city centres at night. (Images: ESA)

Wet Bulb Globe Temperature

Wet Bulb Globe Temperature (WBGT) is similar to the Heat Index in that it is an indicator of the stress on the body from the heat. WBGT was developed in the 1950s after heat related illnesses affected the US armed services during the 1940s. After its implementation, there was a reduction in heat related illnesses during basic training. One fundamental difference between the Heat Index and WBGT is that the latter is calculated with the temperature measured in the sun. While Heat Index only takes temperature and Relative Humidity into account, WBGT takes several variables into account. These variables include:

  • Temperature
  • Humidity
  • Wind Speed
  • Sun Angle
  • Cloud Cover
  • Physical Activity

Heat stress and health in cities

Heat stress is an increasing problem in many European cities due to ongoing climate change. Citizens experience higher levels of heat stress than people living in rural areas, becauseof higher surface and air temperatures in cities, combined with lower wind speeds, and higher levels of solar and thermal radiation coming from buildings.

Daily max. WBGT in the Tivoli Gardens in Copenhagen on 30 June 2019 (source: CURE Project)

Heat stress has a negative impact on sleep, productivity, morbidity and mortality of urban residents. Especially the elderly and young children are vulnerable to heat-related illnesses and death. The EEA reports that 2% of deaths across 15 European cities in the 1990s were attributable to heat. Because of climate change, annual fatalities from extreme heat could rise tenfold by 2050. It is therefore essential to have a good understanding of the factors contributing to heat stress, identifying hot spots in a city, and assessing the effectiveness of adaptation measures.

The CURE Project

To map the heat stress in a city WBGT is a great measurement. The WBGT can be measured easily with rather cheap equipment, making it an ideal indicator for model validation and citizen science campaigns raising awareness about heat stress issues. This is the focus of one of the applications of the CURE project, an Horizon 2020 project that combines Copernicus data from four of the core services with ground-based and third-party data into a dataset specifically aimed at urban resilience in Europe.

In the CURE Project the WBGT is simulated with a spatial resolution of 2m x 2m, based on a very detailed land use map that is calculated from Copernicus Data Warehouse layers. The location of the trees in the cities is added from the Urban Atlas Street Tree layers. The thermal comfort model downscales meteorological conditions from the atmospheric reanalysis of the global climate of the European Centre for Medium-range Weather Forecasts (ECMWF) to the city centre, and combines these with detailed radiation maps, in which the shading effect of buildings and trees is taken into account. The resulting maps depict real live conditions and can be validated against other Earth Observation measurements.

Typical hot summer days

For assessing the produced WBGT maps, the focus is on typical hot summer days, when people suffer from heat stress issues. The CURE AP08 delivers heat stress maps for a typical hot summer day for four European cities: Copenhagen, Ostrava, San Sebastian and Sofia.

Daily mean WBGT in San Sebastian on 23 July 2019 (source: CURE Project)

Both daily maximum and daily mean WBGT maps are calculated from the hourly model output. The daily maximum WBGT maps focus on the hottest hours of the day, when shading is crucial to lower the local heat stress. The daily mean WBGT maps show a time-average of the heat stress situation and also consider the night-time, when the Urban Heat Island (UHI) is at its peak. When assessing the WBGT values, it is good to keep in mind that the values are typically a few degrees lower than regular air temperature values, and a difference of a few degrees can have a strong effect on human thermal comfort.

The importance of trees in the city

From the example maps shown here, it is clear that forested areas are the coolest locations in cities, as the trees provide shade and cool the air through evapotranspiration. On the other hand, open sealed areas without shade from trees or buildings are the hottest locations. Water areas can provide some cooling during the hottest hours of the day, but they often keep a high temperature during the night, aggravating the UHI problem. Planting trees is therefore considered as the most efficient adaptation measure to tackle heat stress problems in a city.

It can be hot in the city! (Image: Pixabay)

The WBGT maps of this Application allow stakeholders (e.g. urban planners and city administrations) to identify hot spots, and give them insight into the local variation of heat stress with a high level of spatial detail. From these maps, overview statistics can be calculated (e.g. city district averages, area above/below defined threshold values).

Furthermore, the Application allows users to modify the input land cover map and upload different land use scenarios, from which new WBGT maps can be calculated instantly. As a result, the users can assess the effectiveness of e.g. green-blue adaptation measures and justify urban adaptation strategies, providing evidence about the impact of these strategies and the related measures on the local urban climate.

Space for Smart Cities Project

Groundstation.Space is part of the Space for Smart Cities consortium. Find out more about this project here.

Remco Timmermans

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