Urban heat island (UHI)

An urban heat island is the pattern in which a city's air and surface temperatures run measurably higher than the surrounding countryside, especially at night, because pavement, roofing, and buildings absorb and slowly release solar heat while vegetation and open soil, which cool through evaporation, are largely absent. The effect can add several degrees to nighttime lows in a dense downtown compared with nearby green or rural areas.

Analysts measure it using ground sensor networks, smart city sensor data, and thermal bands from remote sensing satellite imagery, mapping surface and air temperature across a city to identify the hottest neighborhoods, which are often also the areas with the least tree canopy and the most impervious surface. This differs from a general heat wave, which raises temperatures regionally; the urban heat island is specifically the added, localized warming a city creates relative to its surroundings.

It matters for public health, since heat-vulnerable neighborhoods often overlap with lower-income areas with less tree cover, and it feeds land-use modeling and climate-adaptation planning such as cool-roof mandates and tree-planting programs. A common pitfall is measuring only air temperature at a few weather stations, which misses the fine-grained, block-by-block variation that climate scenario analysis and mitigation planning actually need.