Satellite data shows taller buildings strengthen clouds, while higher density dampens them
Seen from above at dusk, Chicago’s skyscraper grid radiates heat long after the sun sets. A new study reveals that urban skylines influence the clouds that form directly overhead.
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“To produce precipitation, you need clouds,” said Qi Li, an atmospheric scientist at Peking University in China. Clouds influence not only the distribution of precipitation, but also a city’s heat retention after sunset and the effectiveness of rooftop solar panels.
For years, scientists have known that urban areas tend to be warmer than rural areas. The temperature difference can change the lower atmosphere, influencing when and where clouds form. But most previous studies have focused on individual cities or short-term observations.
To look for general trends, Li and his colleagues analyzed nighttime observations collected annually between April and September from 2002 to 2020 by a NASA satellite imaging instrument. In the 44 cities analyzed, urban cloud cover was higher than that of neighboring rural areas, ranging from less than one percent to around 15 percent.
Next, the researchers asked whether city design helped explain this range. Instead of simply averaging the heights of buildings in each city, they grouped each city’s neighborhoods based on their urban design. For example, some areas are dominated by high-rise towers and others by shorter, more sprawling buildings. The team calculated the height of the buildings relative to the width of the streets between them and how closely the buildings are packed together.
Cities with taller buildings relative to street width tend to show greater increases in cloudiness, while cities with more densely populated buildings tend to show smaller increases. The size of the city itself made no measurable difference.
Finally, to understand why this pattern emerged, the team ran computer simulations of hypothetical city blocks, testing how different building heights, densities, and layouts change wind flow, trap heat, and move humidity.
Simulations suggest that taller buildings improve upward air movement, helping warm, humid air rise and form shallow clouds. Densely populated buildings reduce this vertical mixing, which limits cloud formation. At night, cities tend to have less wind, allowing warm air to rise, which is why the increase in clouds was particularly pronounced at night.
Li considers this study a first step. Next comes understanding how changes in clouds translate into changes in precipitation and precipitation extremes.