Are We Measuring Urban Heat Islands Correctly?
| AUTHOR | Dr Y Nithiyanandam |
| DATE | July 13, 2026 |
| CATEGORIES | Remote Sensing Heat Geospatial Research Geospatial Urban Heat |
Over the past few months, I have come across an increasing number of social media posts showcasing colourful thermal maps of cities, claiming to reveal the urban heat island effect. These maps, almost without exception, are derived from satellite-based land surface temperature. While they are visually striking and certainly useful, I often feel we are beginning to equate land surface temperature with the urban heat island itself. They are not the same, and confusing the two can lead to oversimplified conclusions and, more importantly, ineffective mitigation strategies.
There is no denying that land surface temperature is an important indicator. Ever since thermal sensors became available on the Landsat missions in the late 1970s and evolved further, remote sensing has transformed our ability to monitor the Earth’s surface. Today, we can estimate surface temperatures for virtually any city in the world within minutes. This has democratised urban climate studies in ways that were unimaginable a few decades ago. Yet, we should remember that satellites are not measuring temperature in the way most people assume.
A thermal sensor measures the energy emitted by a surface and converts that energy into what we call land surface temperature. What we eventually obtain is the skin temperature of a material, whether it is concrete, asphalt, a rooftop or bare soil. That temperature is influenced not only by sunlight but also by the material itself, heat conducted from below, nearby heat sources and the surrounding environment. In other words, a hot surface does not necessarily mean the surrounding air is equally hot, nor does it always explain why the surface has become warm in the first place.
The urban heat island, in its classical definition, refers to the temperature difference between an urban area and its surrounding rural environment. It is fundamentally an atmospheric phenomenon driven by urbanisation, human activities and the way cities store and release heat. Modern cities, however, are far more complex than the cities on which many of these theories were originally developed. Commercial districts no longer exist only in city centres. Industries have moved to the outskirts. Residential developments have expanded in every direction, while parks, lakes and green spaces interrupt the urban landscape. Instead of one large heat island, cities now contain several localised hotspots and cooler pockets, each shaped by its own microclimate.
This complexity raises an important question. Is land surface temperature enough to understand urban heat? My answer would be that it depends entirely on what we are trying to achieve. If the objective is to identify broad thermal hotspots, compare cities or understand how urbanisation has changed over time, satellite-derived land surface temperature is an excellent tool. It provides consistent observations over large areas and is often the only practical option where ground observations do not exist.
However, if our aim is to reduce heat exposure, evaluate mitigation measures or understand how people actually experience heat, satellite observations alone are not sufficient. We need to measure the atmosphere that surrounds us, not just the surfaces beneath our feet. Dense meteorological networks, smart environmental sensors and carefully designed field measurements remain essential for capturing local variations in air temperature and understanding how heat behaves across different neighbourhoods. These observations tell us whether a newly planted urban forest is actually cooling its surroundings, whether reflective roofs are making a measurable difference or whether a particular intervention has reduced heat stress for the people who live there.
Remote sensing and ground observations should therefore not be viewed as competing approaches. They complement one another. Satellites provide the broader picture, while field observations explain the processes that satellites alone cannot capture. The most reliable understanding of urban heat comes from combining both.
Perhaps the simplest change we can make is also the most important one. Instead of casually referring to ‘temperature’, we should be clear about what we mean. Are we discussing land surface temperature measured from space or air temperature measured near the ground? The distinction may appear subtle, but it makes all the difference when interpreting results and designing solutions.
As cities continue to grow and climate change intensifies extreme heat events, precision in both measurement and language becomes increasingly important. Before concluding that a city has become hotter simply because a satellite image appears redder, we should pause and ask a simple question: what exactly are we measuring? The answer will determine whether we are merely mapping heat or genuinely understanding it.