Abstract
Extreme heat poses significant risks to the world’s growing urban population, and the heat stress to human health is likely to escalate with the anthropogenically increased temperatures projected by climate models. Thus, the additional heat from the urban heat island (UHI) effect needs to be quantified, including the spatial pattern. This study focuses on the city of Valencia (Spain), investigating the intensity and spatial pattern of UHI during three consecutive hot summer days accompanying a heat record. For the analysis, long-term in situ measurements and remote sensing data were combined. The UHI effect was evaluated using two approaches: (a) based on air temperature (AT) time-series from two meteorological stations and (b) using land surface temperature (LST) images from MODIS products by NASA with 1 km resolution. The strongest nighttime UHI estimated from AT was 2.3 °C, while the most intense surface UHI calculated as the difference between the LST of urban and rural regions (defined by NDVI) was 2.6 °C—both measured during the night after the record hot day. To assess the human thermal comfort in the city the Discomfort Index was applied. With the increasing number of tropical nights, the mitigation of nighttime UHI is a pressing issue that should be taken into consideration in climate-resilient urban planning.
Highlights
IntroductionCities are hotspots of climate change due to the increasing frequency and intensity of extreme heat and the rapidly growing urban population, which are mutually reinforcing trends [1]
Cities are hotspots of climate change due to the increasing frequency and intensity of extreme heat and the rapidly growing urban population, which are mutually reinforcing trends [1].The well-documented phenomenon of the urban heat island (UHI) effect refers to cities being warmer than their rural surroundings because of the built environment absorbing, retaining, and/or producing more heat than the natural landscape it replaces [2]
The UHI intensity is greatest at night, and it may disappear by day or the city may be cooler than the rural environments [3]
Summary
Cities are hotspots of climate change due to the increasing frequency and intensity of extreme heat and the rapidly growing urban population, which are mutually reinforcing trends [1]. The well-documented phenomenon of the urban heat island (UHI) effect refers to cities being warmer than their rural surroundings because of the built environment absorbing, retaining, and/or producing more heat than the natural landscape it replaces [2]. The UHI intensity is greatest at night, and it may disappear by day or the city may be cooler than the rural environments [3]. The effect has been mainly described in large cities and towns with high concentration of populations, even rural-villages—small built-up urban areas—can have considerably higher temperatures than their surroundings as a recent study showed [4]. Excessive heat negatively influences human health [11]—including increasing mortality rates due to heat stress [12] and more frequent insomnia events during hot nights [13]—but it has an impact on the labour productivity [14] and the urban metabolism as
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