Mapping Heat in U.S. Cities

We all know that urban areas often feel and are, in fact, hotter than surrounding greener or wooded regions. This is due to the absorbing and re-emitting of heat by concrete and other materials in urban areas relative to natural terrain and greenery. This phenomenon is known as the urban heat island effect.

While this is well known, there are variations within urban areas and this is not always clear as to why and the potential hazards these differences may have on residents. Now, community efforts are trying to map urban heat islands so that hot spots within cities are better known and mitigation can be carried out in an era of increasing heat and temperatures across cities.

Mapping urban heat in North Carolina

In 2021, Raleigh and Durham, North Carolina residents formed a group of over 250 volunteers to map their cities’ temperatures using portable heat and humidity sensors. On a hot July day, these volunteers went about these towns with sensors attached to bikes and cars. The data they recorded helped to map pockets and areas within these cities to help determine where some of the hottest spots might be.^[1] 

This effort is part of a much larger project called the Urban Heat Island (UHI) mapping campaign that sees the mapping of urban heat islands as critical in an era of climate change that is making some cities potentially hazardous.^[2] The National Oceanic and Atmospheric Administration (NOAA) has funded the effort with the consulting group CAPA Strategies leading the effort in organizing residents.

Urban areas are hotter than rural areas

It is estimated 41 million people are vulnerable in the US to much higher temperatures in urban areas, where some cities and areas can be over 8 degrees Fahrenheit over surrounding areas. The idea behind this effort is to obtain very local heat island data so more detailed maps can be created across cities in the US. Residents are organized in local groups and are provided with heat and humidity sensors to record across their cities. 

Going back to the North Carolina efforts, the results of mapping in 2021 have shown and highlighted important lessons on the potential dangers of heat islands in cities. Nearly 100,000 data points were collected in Raleigh and Durham, which yielded results showing up to 9.6 and 10.4 degree difference in Raleigh and Durham respectively for areas within these cities during the summer.

Climate inequality in cities

Such results reflect not only climate inequality within cities, where some neighborhoods are much hotter than others, but legacies of discriminatory urban policies where infrastructure such as parks and greenery efforts were not built in African American communities are highlighted in the mapping effort.

A good example is the Hayti district in Durham where so-called urban renewal in the 1960s led to the construction of a highway right through the heart of a once thriving neighborhood and paving of areas all around; this led to this area now being among the hottest areas in the city due to the excessive pavement found.^[3] 

However, it is not just purposeful legacy that is the problem. Storms and weather contribute to increasing temperatures by killing and damaging trees. For instance, in Oklahoma City in 2020, ice storms led to 90 percent of the tree canopy in the city being damaged, which made the city vulnerable for increasing temperatures within heat islands. 

Using urban heat island data to predict hot spots in cities

Efforts by UHI will be important in not only creating more detailed urban maps that demonstrate potentially dangerous areas within cities and how heat varies within communities, but such effort can now better inform scientists what temperatures might be within specific areas in cities based on heat waves and heat domes that affect given areas.

Satellite data from Sentinel-2 and on the ground mapping are being used to help predict likely hot spots in cities given the configuration of trees, shrubs, concrete, roads, and other infrastructure in specific urban areas.

Using Normalized Built-up Area Index (NBAI) and Normalized Difference Vegetation Index (NDVI) together helps to create very accurate predictions of heat effects and temperature estimates at specified locations in cities. Not only do some areas have higher heat, but it has also been shown that heat lingers longer in areas with less greenery, which results in longer hazardous heat periods for some neighborhoods. These results highlight the need for cities to possibly re-landscape areas within them to make them cooler for residents in times of high temperatures.^[4]

Mapping heat islands within cities will be critical in the future as it can mean the difference between life and death for individuals living in potentially high temperature neighborhoods. These urban areas are often less green and may require mitigation strategies that include urban planning around making city neighborhoods cooler.

We have seen in recent years high heat domes leading to excess deaths in given cities. This trend will likely continue and increase. The need to create more accurate heat island maps in cities is now seen as critical if we are to find out what areas are vulnerable to high temperatures. 

References

[1]    For more on the volunteers in North Carolina mapping heat islands in their cities, see: https://www.nrdc.org/stories/mapping-urban-heat-islands-helping-these-neighborhoods-adapt.

[2]    For more on Urban Heat Island, see:  https://www.heat.gov/pages/mapping-campaigns.

[3]    For more on results showing different heat islands within Raleigh and Durham, see:  https://cityofraleigh0drupal.blob.core.usgovcloudapi.net/drupal-prod/COR27/HeatWatchRaleighDurhamReport2021.pdf

[4]    For more on using satellite and ground data to map and forecast heat islands in urban areas and estimate temperatures, see:  Shandas V, Voelkel J, Williams J, et al. (2019) Integrating Satellite and Ground Measurements for Predicting Locations of Extreme Urban Heat. Climate 7(1): 5.