The interplay of cloud cover and 3D urban structures reduces human access to sunlight

Sunlight exposure in urban areas is essential for human health, yet its reduction due to cloud cover and complex 3D urban architecture is often overlooked. Here we introduce a framework that combines natural day length variations, cloud cover and 3D urban structures to assess sunlight duration in cities. Applying this framework to 1,353 US cities, we found a significant decrease of 2,896 h (121 days) in available sunlight for 2020, with cloud cover accounting for 2,448 h (102 days) and urban structures contributing to a 448-h (19-day) loss. With the observed increasing trends in cloud cover and urbanization, sunlight loss is likely to worsen. Our study highlights the pressing need for interdisciplinary urban planning strategies that prioritize adequate access to natural sunlight.