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Hong Kong has been experiencing hotter summers and more extreme hot days in recent years due to climate change and the heat island effect. A study from the Chinese University of Hong Kong (CUHK) found that hot nights pose a greater threat to public health than hot days. Specifically, researchers found that consecutive hot nights are more detrimental to human health than very hot days, although the actual temperature does not reach daytime levels.
Apparently, consecutive hot nights brought more health problems compared with very hot days, especially for five or more consecutive hot nights. It was also found that when consecutive very hot days were joined with consecutive hot nights, such as two consecutive very hot days with three hot nights, the health impact was significantly amplified, compared with only consecutive very hot days. Females and older adults were found to be relatively more vulnerable to extreme hot weather.
According to Kevin Ka-Lun Lau from the Institute of Future Cities at CUHK, "Nighttime is supposed to provide the body with a chance to recover and rest from the heat of the day, but hot nights make the recovery and resting less effective. Due to the congested living environment with poor air ventilation, the occupants in sub-divided flats are particularly vulnerable and not able to recover from the heat of the day.”
The research also identified the lack of urban greenery and poor air ventilation in a high-density context as factors that contribute to more hot nights than hot days in some areas. The team suggests better urban planning and building design as long-term mitigation measures. They also argue that improved city and indoor natural ventilation and an increased greenery ratio would help to mitigate higher air temperatures and could improve public health.