Out of the light
A new study led by Dr Bin Chen at the University of Hong Kong has revealed the extent to which cloud cover and three-dimensional urban structures reduce sunlight exposure in cities, potentially impacting public health and energy management. The research, published in Nature Cities with Dr Shengbiao Wu as the first author, analysed 1,353 US cities for the year 2020.
Sunlight is essential for public health, influencing vitamin D production and psychological well-being. Insufficient sunlight exposure is linked to cardiovascular diseases, metabolic syndrome, depression, and mental disorders.
The study found that the potential annual sunlight exposure of approximately 4,460 hours is drastically reduced by an average of 2,896 hours (equivalent to 121 days) in urban areas. Cloud cover accounts for the majority of this reduction, causing a loss of 2,448 hours (102 days), while urban structures contribute to an additional 448-hour (19-day) decrease.
The project underlines the importance of capturing the complex interplay between geographic, climatic, and structural elements affecting urban sunlight exposure. The research team developed a framework that combines natural day length variations, cloud cover, and 3D urban structures to assess sunlight duration in cities, creating a “digital twin” of the urban environments.
The findings highlight geographic variations in sunlight reduction, with northeastern and northwestern regions of the US experiencing more pronounced effects from cloud cover. Coastal cities face more severe impacts from urban shading.