Urban Heat Hot Spots - Climate Central

Urban Heat Hot Spots | Climate Central

More than half of the global population and about 80% of the U.S. population lives in cities. Urban populations tend to experience higher average temperatures and more intense heat extremes than people in less developed areas.

This is because, in addition to the primary warming from carbon pollution, cities also experience an extra temperature boost due to the urban heat island effect.

During extreme heat events such as this summer’s relentless heat waves in the southern U.S., the urban heat island effect can worsen heat stress and related illness for millions, put vulnerable populations at risk, and lead to higher energy bills and strained power grids during spikes in cooling demand.

According to the U.S. Environmental Protection Agency,

“The urban heat island effect is a measurable increase in ambient urban air temperatures resulting primarily from the replacement of vegetation with buildings, roads, and other heat-absorbing infrastructure. The heat island effect can result in significant temperature differences between rural and urban areas.”

Most of the planet is warming due to human-caused climate change, but the built environment in cities amplifies both average temperatures and extreme heat. Extreme heat is the deadliest natural hazard in the U.S. Children and adults over 65 are among those most vulnerable to heat-related illness.

Estimating urban heat intensity

Climate Central analyzed the urban heat island (UHI) index in 18,945 census tracts comprising 44 large U.S. cities in order to understand where urban heat is most intense within each city. The combined population of all 44 cities analyzed is over 74 million—about 22% of the total U.S. population.

The UHI index in this analysis is based on a study, Sangiorgio et al. (2020), that quantifies the factors that cause developed areas to heat up. Climate Central adapted the modeling approach in Sangiorgio et al. (2020) using data on the distribution of land cover types in each city (from green space to paved areas) as well as building height and population density, to estimate how urban heat island intensity varies within each of the 44 cities included in this analysis. See Methodology below for more detail.

The primary factors that influence UHI index estimates in this analysis are, in order of influence: albedo, percentage of green space, and population density. Other characteristics of the built environment, including the width of streets, orientation of urban canyons, and building heights have a smaller relative influence on UHI index estimates. See Why it’s hotter in the city below to learn more about each factor.

Of all these factors, albedo generally has the largest effect on UHI index values.
Albedo is the proportion of incoming sunlight (solar radiation) reflected by a surface. Roads, sidewalks, buildings, and parking lots all have low albedo, meaning that these common urban surfaces absorb more sunlight and radiate it back into the city as heat. Water bodies also have low albedo; in coastal cities, low-albedo water bodies can drive higher UHI index values—as seen in Seattle, New Orleans, and Miami among others.

The UHI index values (in °F) in this analysis are estimates of how much the urban built environment boosts the temperature in a census tract. The UHI index is an estimate of the additional heat that local land use factors contribute to urban areas.

For example, on a 95°F day in rural northern Texas, people living or working in a downtown Dallas area with a UHI index value of 9°F would experience temperatures of at least 104°F.

This analysis differs from a 2021 Climate Central analysis in methodology and scope. The primary aim of this analysis is to assess how urban heat intensity varies within 44 major U.S. cities. By contrast, the 2021 analysis determined a single average UHI index value for each of 158 U.S. cities, but did not evaluate the relative heat intensity of different areas within each city.

Mapping heat hot spots within 44 cities

Urban heat intensity and patterns of high heat exposure vary within and across the 44 cities analyzed.

Maps of each city reveal how different land use and urban growth patterns influence the distribution of heat hot spots across census tracts—whether concentrated in a small urban core or sprawling across a vast area.

In some cities, heat intensity is concentrated in a distinct urban core. These cities have an extreme apparent contrast in UHI index values between the urban core and surrounding less-developed areas:

Albuquerque, Bakersfield, Colorado Springs, Fresno, Indianapolis, Jacksonville, Las Vegas, Oklahoma City, Philadelphia, Sacramento, San Antonio, San Jose, Tulsa, and Wichita.

Other cities have more diffused zones of heat intensity, with a smaller relative difference in UHI index values between census tracts in the urban core and outlying areas. These patterns are apparent in:

Atlanta, Baltimore, Boston, Chicago, Denver, Milwaukee, and San Diego.

Still other cities are dominated by sprawling heat intensity. In these cities, high UHI index values are not concentrated in a central core but rather spread across a vast developed land area:

Dallas, Detroit, Houston, Los Angeles, Memphis, Miami, Newark, New Orleans, New York City, Omaha, Phoenix, Portland, Seattle, and Washington, D.C.

How many people experience extreme urban heat?

UHI index values for individual census tracts ranged from 4.7°F (in Denver) to 13.2°F (in New York).

The most extreme 25% of all 18,945 census tracts analyzed had UHI index values of 8.1°F and higher.

In order to assess how many people live in areas with extreme UHI index values, Climate Central analyzed the total urban population living in census tracts at 8°F and above.

Download data to explore population exposure at a range of other UHI index thresholds.

Across all 44 cities, the total population living in census tracts with a UHI index of 8°F or higher is 41 million—or about 55% of the 74 million people included in this analysis.

In other words, the urban population included in this analysis is concentrated in census tracts with higher-than average urban heat intensity.

More than two-thirds of residents in the following cities experience at least 8°F more heat due to the urban heat island effect: Detroit (86%), New York (78%), Dallas (75%), New Orleans (74%), Houston (73%), Portland (67%), San Antonio (67%), and Omaha (66%).

Nine cities included in this analysis had at least 1 million people exposed to an UHI index of 8°F or higher—meaning that on a day when temperatures in a park outside the city are 90°F, it feels like 98°F or higher for at least 1 million people in the city.
City Population with UHI index of 8°F or higher Percent of total population (%) New York 7,124,781 78 Houston 4,254,195 73 Los Angeles 3,271,985 45 Dallas 2,199,962 75 Chicago 2,098,100 52 San Antonio 1,461,288 67 San Diego 1,359,138 55 Phoenix 1,318,985

45
Detroit 1,017,182 86

City-wide urban heat rankings

The UHI index values for individual census tracts were averaged by each city’s total area and population to determine average UHI index values for all 44 cities.

The cities with the highest average UHI index values are distributed across the majority of U.S. climate regions—with the exception of the Southwest and the Northern Rockies and Plains.

The city-wide average UHI index by area ranged from 5.8°F in Las Vegas to 8.6°F in New York.

City
Average UHI index, area-weighted (°F) U.S. climate region

New York
8.6 Northeast Newark 8.4

Northeast
Miami 8.3 Southeast Seattle 8.1 Northwest New Orleans 8.0 South

Detroit

8.0
Upper Midwest

Chicago
7.9 Ohio Valley Minneapolis 7.8

Upper Midwest
San Francisco 7.7 West Portland

7.7

Northwest
Boston

7.7

Northeast

Dallas

7.7

South
Baltimore

7.7

Northeast
The average UHI index per capita was lowest in Wichita (7.2°F) and highest in New York (9.5°F), San Francisco (8.8°F), Chicago and Miami (8.3°F), and Seattle (8.2°F).

In 39 of the 44 cities analyzed, the population-weighted average UHI index values were higher (by 0.1 to 1.6°F) than their area-weighted average UHI index.

City
Average UHI index, population-weighted (°F)

U.S. climate region

New York
9.5

Northeast

San Francisco
8.8

West

Chicago

8.3

Ohio Valley

Miami

8.3

Southeast

Seattle
8.2

Northwest

Los Angeles

8.1

West

New Orleans

8.0

South
Washington, D.C.

8.0

Northeast

Newark

8.0

Northeast

Boston

8.0

Northeast
Detriot

8.0

Upper Midwest

Baltimore

8.0

Northeast
Philadelphia

8.0

Northeast

Who lives in urban heat hot spots?

Structural inequities can lead to higher urban heat exposure in some communities. According to a 2021 study, people of color and those living below the poverty line are disproportionately exposed to urban heat island intensity in 169 of the largest U.S. cities.

Similarly, a 2021 report from the U.S. Environmental Protection Agency (EPA) found that, with continued climate warming, several populations face disproportionate exposure to extreme heat in 49 major U.S. cities. The EPA found that “Black and African American individuals are 40% more likely than non-Black and non-African American individuals to live in areas with the highest projected increases in extreme temperature related mortality with 2°C of global warming.”

The same analysis also found disproportionate exposure to extreme heat among low income (+11%) and Hispanic or Latino (+1%) populations relative to reference (e.g., non-low income) populations.
Why it’s hotter in the city

Several key factors that drive extreme heat in cities were included in the UHI index:

Miles of hard, dark surfaces absorb and then radiate heat back into cities. Albedo is the proportion of incoming sunlight (solar radiation) reflected by a surface. Of all the factors used to calculate UHI index values, albedo generally has the largest influence.

Population density matters.Heat emissions can come from many aspects of urban life including transportation, industrial facilities, and the heating and cooling of buildings. During a summer heat wave, air conditioning from urban buildings can add 20% more heat to the outside air.

There are a number of short-term and long-term solutions to adapt to an increasingly warmer future, and to mitigate urban heat stress and related illness. Short-term solutions are primarily about getting people out of extreme or dangerous levels of heat and ensuring their health and safety.

There are also ways to reduce urban heat island effects over the long-term such as:
A green roof, or rooftop garden, is a vegetative layer grown on a rooftop that can provide shade and reduce rooftop temperatures. Cool roofs are made of highly reflective (i.e., high albedo) and emissive materials that remain cooler than traditional materials, and help to reduce energy use. Cool pavements are an alternative to conventional concrete or asphalt sidewalks and roads, which can reach peak summer temperatures of 120–150°F and radiate that heat contributing to the nighttime urban heat island effect. Cool pavements are reflective and/or permeable materials that help reduce surface temperatures. Research and development is ongoing on cool pavement types and their benefits and tradeoffs.

LOCAL STORY ANGLES

How is extreme heat affecting public health near you?

The Centers for Disease Control and Prevention Heat and Health Tracker maps heat-related illnesses at the census-tract level in real time. Use the National Integrated Heat Health Information System (NIHHIS) Heat Equity Mapper to find census tracts in your area with the highest heat burden. Check the U.S. Department of Health and Human Services monthly Climate and Health Outlook for extreme heat outlooks in your region throughout the summer.

How are local officials protecting people from extreme heat?

Angel Hsu, PhD Assistant Professor University of North Carolina Relevant expertise: urban heat island exposure Contact: angel.hsu@unc.edu

Vivek Shandas, PhD (he/him/his) Professor Portland State University Related expertise: Exposure to intra-urban heat, air pollution-heat dynamics, health impacts, social/infrastructure vulnerability assessments, public policy around climate change Contact: vshandas@pdx.edu

Diego Manya Research Associate Data-Driven EnviroLab Related expertise: Urban heat island inequities in U.S. cities Contact: dcmg2005@gmail.com *Available for interviews in Spanish and English

Daniel J. Vecellio, PhD Postdoctoral Research Scholar, Virginia Climate Center George Mason University Related expertise: Extreme heat and impacts on human health Contact: dvecelli@gmu.edu

FIND EXPERTS

Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.

Population exposure estimates to UHI index values of various thresholds are based on 2020 Census data at the census tract level. City-wide UHI index values are averages of all census tracts within each city, weighted either by total area or by total population.