Urban Heat Islands: How USGS and EPA Track Hotter Cities From Space

Sioux Falls in a Landsat image, 2017 (bands 6, 4, 3). U.S. Geological Survey.

What an urban heat island is

Urban heat islands (UHIs) are built-up areas hotter than the countryside around them. Buildings, roads and other structures absorb and re-radiate the sun's heat more than forests or water do. How strong a heat island is depends on population, urban extent and the local climate.

• Health: heat islands affect city dwellers profoundly and can raise the risk of heat-related death as the climate changes. Heat waves, among the most damaging weather extremes, strain health, energy demand and ecosystems.
• The trend: as Earth warms, hotter days and nights are becoming more common and heat waves more frequent and intense, and heat islands are expected to strengthen as cities grow and densify.
• Why surface temperature: changes in land surface temperature can disrupt natural processes, especially when they outpace how fast plants and animals can adapt, and urban temperatures reveal the effects of human activity and heat stress.

The U.S. Geological Survey (USGS) has imaged Earth's land surface continuously through the Landsat satellites. Landsat surface temperature is key to studies of Earth's energy balance and useful for monitoring heat islands.

The project

A USGS–EPA effort combines Landsat surface temperature with yearly land cover change from the 1980s to today (2021) to measure heat-island intensity and trends anywhere in the country. It also pinpoints heat patterns at the urban–wildland edge and hotspots vulnerable to intense heat waves. National Oceanic and Atmospheric Administration scientists are interested too — for example, in how urban sprawl affects climate records.

How intensity is measured

A system at the USGS Earth Resources Observation and Science (EROS) Center processes every Landsat surface-temperature record into yearly means and trends, maps urban and surrounding non-urban land cover, and measures the heat island as the temperature difference between them. High-intensity urban land is usually hottest.

Land surface temperature by land cover type, from crops and forest through low-, medium- and high-intensity urban land to water and wetland. U.S. Geological Survey.

Comparing urban and non-urban temperatures within a 5-kilometer buffer, the team calculated heat-island intensity and trends from 1985 into the 2010s in five test cities: Atlanta, Houston, Minneapolis, Sioux Falls and Phoenix.

• Most urban pixels were warmer than their surroundings.
• Intensity rose in four cities, by 0.2–0.6 degree per decade, but fell slightly in Phoenix.
• The rise was clearer in warm, wet climates than in dry ones.

Urban areas of the conterminous United States and the five test cities, with an inset of urban–rural temperature differences (2016) and mean yearly heat-island intensity, 1985–2018. U.S. Geological Survey.

The approach can be applied to other major U.S. cities.

The products

The main product is an operational dataset — GIS layers and trend analyses of heat-island intensity and distribution, currently 1985–2018 — published as a USGS data release (Xian and Shi, 2021) from the EROS Center. The datasets will keep coming for major cities, feeding the U.S. Global Change Research Program and EPA climate indicators.

Sources

Based on Monitoring and Assessing Urban Heat Island Variations and Effects in the United States, U.S. Geological Survey Fact Sheet 2021–3031, by George Z. Xian, prepared with the U.S. Environmental Protection Agency, USGS Publications Warehouse; a work of the United States government in the public domain. Figures recovered from the fact sheet's PDF; a high-resolution aerial image of Sioux Falls, of unstated source, is not reproduced.