A researcher attached a mercury thermometer to his car door and drove around Vienna for three hours to record temperatures. His maps showed hot areas that were close to the inner city and cooler areas that were close to the woods. The first map of a city's "islands" of heat in a "sea" of lower- temperature surroundings was created bySchmidt.

Fifty percent of heat-related deaths in parts of England were linked to islands during Europe's 2003 heat wave. The Environmental Protection Agency says that these zones are a major contributor to the number of heat-related deaths in the United States. More than half of the world's population now lives in cities, which make it more likely that global temperatures will rise.

Eva Marques and her colleagues at the University of Toulouse are using modern methods to chart dangerous heat zones. Their approach uses thermometers in Internet- connected personal cars to map how temperatures can vary over just a few city blocks; this data could help planners develop heat-mitigation policies in places without access to sophisticated instrumentation.


When natural land cover is replaced with asphalt, concrete, steel, or other materials that absorb and retain more heat than their surroundings, an urban heat island is formed. This helps keep these areas warm at night. The air quality in a city is affected by the influence of heat islands. City planners need to rethink how urban spaces are designed due to the increase of extreme events.

Many cities don't have weather station networks that can monitor heat islands comprehensively, so Marques and her colleagues took advantage of internet- connected sensors that are becoming standard equipment in cars. In order to compare the accuracy of the car-mounted thermometers with the high-resolution weather stations in Toulouse, the researchers collected data from the car temperature sensors. The scientists used a database of millions of car sensor measurements collected for insurance purposes to create temperature maps in several western European cities.

Heat map of Toulouse, France.
The urban heat island surrounding Toulouse, France, is shown in aggregated car thermometer data from 2018 summer nights. Warmer colors indicate hotter air. Credit: University of Toulouse, Météo-France/CNRS, France

The researchers were able to estimate temperature variations for spaces as small as 200 by 200 meters. They can use this method to assess heat at the street level. The work was detailed in a bulletin.

A research scientist at the NCAR was not involved in the study. The work shows a new use for the data. Iain Stewart, a researcher in urban climatology at the University of Toronto who was not involved in the study, said that the work was provocative and pointed to future possibilities for data retrieval in cities.

Weather and climate data can be provided by factory-installed sensors in vehicles. Ensuring data consistency and quality is one of the challenges. The NCAR collects vehicle data and uses a system that updates every five minutes. Many of the projects use a lot of dedicated vehicles, which don't provide adequate coverage unless they are in a very confined area. A scale-up is needed to move beyond pilot studies. Quality control is important for collecting data from cars that are in an area.


Those capabilities are being worked upon by authorities. In the U.S., local, state and national initiatives aim to install and operate infrastructure that can collect and process connected vehicle data. There are pilot programs in New York and Wyoming that monitor traffic and weather. There are gaps between fixed weather stations that can be filled with such data.

Researchers could use our maps to better understand how new buildings and bodies of water impact thermal variations. She wants to make the data available for policy making. Toulouse municipal officials work with her team to make schoolyards green and prioritize neighborhoods for improvements to cool buildings more efficiently, although they are not yet using her maps. Some small French cities that don't have sophisticated weather station networks still want to use heat maps. Crowdsourced data is a new hope to produce and share maps with these cities in the future.

It is difficult to create temperature maps of sufficient quality for city planning due to the early stages of data gathering. Maps should include places where people gather. Stewart says that hot, crowded cities in lower-income regions of the world stand to benefit most. Many lower-income cities in tropical regions don't have access to the instruments that help other parts of the world and are vulnerable to urban heating.

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Planners could use more detailed maps of these cities to plan for climate change. Stewart says that the advantage is that cars are everywhere. You have the entire world at your disposal.