Urbanization Magnified Nighttime Heat Waves in China

Nighttime heat waves have greater impacts on human society than daytime because nighttime heat waves deprive humans to recover from daytime heat and increase energy consumption for cooling. In this study, we found increased occurrence and severity of nighttime heat waves across China during 1980–2017 based on measurements from more than 2,000 meteorological stations. The nighttime heat waves have been longer lasting and occurred more often in spring and fall. Compared to rural areas, urban areas have shown enhanced frequency, intensity, and duration of nighttime heat waves. Urbanization accounted for nearly 50% of the extended duration and nearly 40% of the enhanced intensity and frequency of nighttime heat waves in urban areas relative to rural areas. Urban expansion, causing reduced evapotranspiration and weakened wind speed that normally cools the lower atmosphere by turbulent heat loss and cooled air advection, led to nighttime urban heat island, thus magnifying nighttime heat extremes. Plain Language Summary Extreme temperature events will likely increase and cause severe damage to human society and natural ecosystem under climate change and urbanization. Compared to daytime, nighttime heat waves reduce people's ability to cool off and prevent the human body recovering from daytime heat exposure, and therefore increase the risks of heat illnesses and deaths. Here, we show that nighttime heat waves have been more frequent, longer lasting, and severer, and occurred increasingly in spring and fall in China. These changes have been more intensive in urban areas than their surrounding rural areas. Urbanization accounted for nearly 50% of the extended duration, and nearly 40% of the enhanced intensity and frequency of nighttime heat waves in urban areas relative to rural areas. Nighttime urban heat island due to rapid urban expansion magnified nighttime heat extremes. Key Points Nighttime heat waves have been more frequent, longer lasting, and severer, and occur increasingly in spring and fall across China since 1980s Urbanization accounts for ∼50% of the extended duration and ∼40% of the enhanced intensity of nighttime heat waves in urban relative to rural areas Urbanization reduces evapotranspiration and weakens wind speed thereby leading to urban heat island that magnified nighttime heat waves