Persistent cloud cover over mega-cities linked to surface heat release

Urban areas are a hotspot for the interactions between the built environment, its inhabitants, and weather. Unlike the impact of temperatures through the well-known urban heat island effect, urban effects on cloud formation remain unknown. In this study we show observational evidence of a systematic enhancement of cloud cover in the afternoon and evening over two large metropolitan areas in Europe (Paris and London). Long-term measurements in and around London show that during late-spring and summer, even though less moisture is available at the surface and the atmosphere is drier, low clouds can persist longer over the urban area as vertical mixing of the available moisture is maintained for a longer period of time, into the evening transition. Our findings show that urban impacts on weather extend beyond temperature effects. These prolonged clouds over the city might enhance the urban heat island via night-time radiative forcing. Urban climato l ogy: A cloudy picture for hot cities Compared to nearby rural areas, mega-cities can produce an atmosphere that is, paradoxically, drier yet persistently more cloudy. Cities are known to create an “urban heat island”, but a multi-institution team led by Natalie Theeuwes from the University of Reading now shows that the urban fingerprint extends to an increase in cumulus cloud cover. In an analysis of the Paris and London mega-cities, the team finds that afternoon cloud cover during spring and summer is, on average, several percent higher over the urban core than over surrounding rural areas. The results are initially counter-intuitive, because reduced vegetation in cities also tends to dry the atmosphere, which should reduce cloud cover. By probing detailed ground-based observations in and near London, the researchers explain that buildings release their heat throughout the afternoon, enhancing turbulent mixing and thus the delivery of moisture to clouds. The findings illuminate the widening impact of cities on their environment.