Analyzing Surface Urban Heat Island Trends and Regional Variability Across 150 U.S. Cities (2000–2024) Using MODIS Data and a Reproducible Python Application

Abstract

Urban Heat Islands (UHIs), or Surface Urban Heat Islands (SUHIs), occur when urban surfaces register measurably higher temperatures than surrounding non-urban land, a consequence of replacing natural vegetation with impervious materials that absorb and re-emit solar radiation. As more than 80% of the U.S. population now lives in urban areas, the thermal consequences of continued urbanization carry growing implications for public health, energy demand, and climate adaptation planning. This study analyzes SUHI dynamics across 150 U.S. cities from 2000 to 2024 using MODIS land surface temperature data, an automated Google Earth Engine (GEE) data extraction application, and multi-year trend analysis. Three questions are addressed: (i) whether an open-source, GEE-based Python application can reliably automate consistent SUHI extraction for any U.S. city; (ii) how regional SUHI trends and temporal trajectories differ across five U.S. geographic regions and three city-size classes over the 25-year study period; and (iii) how the Diurnal Asymmetry Index (DAI) identifies whether cities have stronger daytime or nighttime SUHI and how these differences are related to regional hydroclimate conditions. Our results show that urban heat trends are not the same across the United States. Some regions experience much stronger SUHI changes than others. We also found that after around 2018, the strengthening of SUHI generally slowed in many cities, although the amount of slowdown varied by location. Another key result is that daytime and nighttime SUHI behavior separates into two distinct patterns, and those patterns align closely with climate zones defined by the Köppen–Geiger classification. Finally, the automated system successfully produced standardized and reproducible annual metrics for all 150 cities, showing that large-scale SUHI monitoring can be done in an open and accessible way.