Study reveals limits of using land surface temperature to explain heat hazards in Miami-Dade County

A recent study published in the journal PLOS Climate on October 2, 2024, examines the effectiveness of using land surface temperatures (LSTs) as proxies for surface air temperatures (SATs) in subtropical, seasonally wet regions. Scientists at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science, used satellite remote sensing data to explore how LST reflects human heat exposure in Miami-Dade County, Florida. The findings have important implications for urban heat adaptation strategies, raising questions about how well LST captures the full extent of outdoor heat exposure in this region and beyond.

Land Surface Temperature as a Proxy for Heat Exposure

"LST data, gathered by satellite imaging, have long been used to estimate surface air temperature -- the temperature people experience outdoors," said Nkosi Muse, a Ph.D. candidate in the Abess Graduate Program in Environmental Science and Policy at the Rosenstiel School and the lead author of the study. "LSTs are a key component of lower atmosphere processes and can be studied at high resolutions -- important for understanding urban heat risks and informing adaptive strategies, especially as cities grow hotter due to climate change and urban development," he notes.

The researchers indicate the accuracy of LST as a proxy can vary based on geographical and climatic factors. While widely studied in temperate zones, the relationship between LST and SAT in subtropical regions with high summer rainfall remains less explored.

This study, focused on Miami-Dade County, aimed to bridge this gap. Using Landsat 8 remote sensing data from 2013 to 2022, researchers compared LST readings with air temperature data from local weather stations to understand when and where LST is an effective proxy for SAT. Their findings revealed seasonal variations in the relationship between LST and SAT, underscoring the complexity of using LST data in subtropical, wet regions.

Seasonal Patterns of LST and the Urban Heat Island Effect

The study found that LST data captured the spatial distribution of heat across the county, notably highlighting the presence of a surface urban heat island (SUHI) effect -- where urban areas are hotter than surrounding rural areas. This effect was most pronounced during spring, with a mean SUHI intensity of 4.09°C, surprisingly higher than during the summer when it averaged 3.43°C. Notably, LST peaked in May and June, contrary to the typical northern hemisphere pattern where summer months like July and August tend to see the highest temperatures.

In contrast, SAT in Miami-Dade County reached its highest levels in August, with the relationship between LST and SAT varying significantly by season. During winter, LST closely aligned with SAT, but this connection weakened during wetter fall months. In summer months, there was no statistically significant relationship between LST and SAT.

Limitations of LST as a Heat Exposure Measure

While LST remains a useful tool for identifying spatial heat patterns in urban areas, this study suggests its limitations as a proxy for the air temperatures people experience in subtropical, seasonally wet regions like Miami-Dade. During the wet season, LST may underestimate the actual heat exposure residents face. The timing of LST data collection (11 AM ET/12 PM EST) might also play a role, as this snapshot does not capture the peak heat of the day, especially during humid, rainy months.

"These findings highlight the risks of relying solely on LST for urban heat adaptation strategies, especially in climates that do not follow temperate patterns," said Amy Clement, a professor of atmospheric sciences at the Rosenstiel School and a co-author of the study. "As cities around the world, particularly in subtropical and tropical regions, face increasing threats from heatwaves and rising temperatures, these results emphasize the need for more precise measurements to accurately assess heat risks and inform responses," she says.

Implications for Urban Planning and Future Research

The study's findings have immediate relevance for urban planners and policymakers working on heat adaptation strategies in subtropical and tropical regions. As Miami-Dade County continues to develop new heat policy and the City of Miami unveils its first ever "Heat Season Plan," these findings can be incorporated into further planning. The findings suggest that relying on LST alone may lead to a misrepresentation of heat risks, particularly during the wet season, when air temperatures can be significantly higher than surface temperatures.

As urban areas face growing pressure to protect at-risk populations from extreme heat, this research points to the need for more sophisticated approaches to measuring and mitigating heat exposure in Miami-Dade. Using LST data to identify neighborhoods most at risk from heat may overlook the intensity of heat exposure in some areas, particularly during the hottest months of the year. This could result in inadequate or misdirected heat adaptation strategies.

The study also opens avenues for future research, particularly in exploring how localized processes -- such as vegetation, water bodies, or urban materials -- affect surface energy balances and LST readings. Understanding these factors could improve the accuracy of LST as a tool for measuring heat exposure in diverse urban environments.