While it is clear that the planet is getting hotter in a lot of locations, particularly our cities which absorb and re-emit a lot of heat in the concrete materials we use to build them.
In some cases, this is just some discomfort, but for some of our hottest cities this could lead to serious health concerns, including death. In fact, recent studies have indicated that, among natural disasters, heat is the leading cause of death in the United States.
Monitoring Urban Heat with Remote Sensing
Increasingly, our satellites are being used to monitor heat not only on urban heat islands but also on farms and other areas that are being impacted by increasing temperatures. Nevertheless, results show we are most vulnerable to heat in our urban regions and this is where we might have to put greater effort to mitigate heat impacts in the near future.
The most common way for using our satellites to determine temperatures is thermal mapping.
Optical data could even be used as land use effects from increasing heat become evident on land surfaces, such as wilted crops.
Mapping Urban Heat Islands
In a recent study, using MODIS Aqua data that measures land surface temperature/emissivity, researchers from the European Commission’s Joint Research Centre compared cities with over 50,000 people and rural areas surrounding these cities.
They found that cities often show 10-15°C higher temperatures than their immediate surroundings. While generally cities are hotter than their surroundings, in desert regions it was found cities are somewhat cooler, probably because more vegetation is found in cities surrounded by desert conditions such as in Cairo.
The study was able to use relatively high resolution data, which allowed it to also differentiate different parts of cities and their temperatures. The findings show that intra-city variability in temperature is high, with industrial areas, areas with dark construction materials, and areas that had a complete lack of vegetation being the hottest zones within cities.
Bodies of water and parks often serve as temperature mitigation within cities. Highly concentrated areas of urban build-up that leads to greater heat is often referred to as urban heat island (UHI).
Mapping Areas that are Vulnerable to High Temperatures
Other research has also used Landsat 8/TM/ETM+ data to measure surface temperatures and create vulnerability indices to help measure how dangerous heat can become in urban and rural regions. In fact, researchers have created land surface temperatures (LST) that are calculated based on the density of features such as roads, buildings, vegetation, and population density that are measured from thermal instruments onboard Landsat.
Companies, third sector, and different enterprises have tried to fill the gap and need for more accurate reporting on when LST reaches potentially dangerous levels for people. In the UK, a private company called 4 Earth Intelligence, used the ordinance survey and satellite data to create detailed maps of all of Britain (30-m resolution) that can now show Heat Hazard Index (HHI) scores using mainly Landsat 8 data.
The scores range between 1-5, with 5 being the most hazardous score for heat. While this uses archival data mainly to derive the scores, there is potential to make it real-time and is now being used by local communities and councils to mitigate the most vulnerable areas that generally have high HHI scores when heat waves occur.
This type of data allows councils to even focus on specific buildings or neighborhood blocks that are the most vulnerable to high heat.
Earthdata Search Tool
To make use of the varied data satellite instruments, particularly MODIS and Landsat, that have captured vital weather information and other data over many years and seasons, NASA has created the Earthdata Search tool.
This tool combines different satellite data to provide information that is easier to determine and calculate such as LST and air surface temperature rather than having to search and calculate these measures independently. The tool also combines socio-economic and population data so that it can be used by planners to better map and determine which areas are likely to face greater threat in heat waves.
Results by researchers show that over the last twenty years, our cities are becoming much hotter than their surroundings and we will have to find ways to mitigate these variations if we are to make some of the most vulnerable cities more habitable. Indices are being used to find the most vulnerable areas, particularly within cities.
Tools, such as Earthdata Search, help to combine different imagery and provide the information needed for decision makers. Far more work will be needed, particularly in better preparing our cities for temperature extremes, but planners now have the tools needed to find the most vulnerable areas to high temperatures within given regions.
 For more on measuring global cities and rural regions, including their temperature differences, see: Mentaschi, Lorenzo, Grégory Duveiller, Grazia Zulian, Christina Corbane, Martino Pesaresi, Joachim Maes, Alessandro Stocchino, and Luc Feyen. 2022. ‘Global Long-Term Mapping of Surface Temperature Shows Intensified Intra-City Urban Heat Island Extremes’. Global Environmental Change 72 (January): 102441. https://doi.org/10.1016/j.gloenvcha.2021.102441.
 For more on estimating land surface temperatures from different land use features, see: Li, Ying-ying, Hao Zhang, and Wolfgang Kainz. 2012. ‘Monitoring Patterns of Urban Heat Islands of the Fast-Growing Shanghai Metropolis, China: Using Time-Series of Landsat TM/ETM+ Data’. International Journal of Applied Earth Observation and Geoinformation 19 (October): 127–38. https://doi.org/10.1016/j.jag.2012.05.001.
 For more on HHI and how it can be used for planning and mitigating heat-related threats, see: https://www.ordnancesurvey.co.uk/newsroom/blog/4ei-spatial-data-can-help-society.