Cities in the Clouds

QUESTION: We know cities can influence air temperatures through the urban heat island effect, but do they impact the atmosphere in other ways?

ANSWER: According to a study published recently in the Proceedings of the National Academy of Sciences, clouds are more likely to appear above urban areas than surrounding rural locations.

Researchers looked at satellite images collected between 2002 and 2020 by the Moderate Resolution Imaging Spectroradiometer on NASA’s Aqua satellite to analyze cloud cover over 447 different U.S. cities with populations of at least 50,000 as well as neighboring areas outside the cities. They discovered that city skies had 3%–6% more cloud cover during summertime than rural skies, and summer nights over cities were twice as cloudy as urban summer days (the increased cloud cover was not observed during winter). The researchers believe the effect could be caused by heat rising from cities drawing moisture from the surrounding landscape and creating clouds. Increased cloud cover over inland cities during summer days and greater cloud cover over coastal cities during summertime nights was possibly because of limited wind circulation over the hotter urban infrastructure. In future research, they plan to study how air pollution influences clouds. “We really care how urban environments impact the overall long-term trends of the atmosphere,” says lead author Thuy Trang Vo, a doctoral candidate at the University of Alabama in Huntsville. [source: Eos]

* For more content from the Bulletin of the American Meteorological Society, please click here.

Photo credit: iStock.com/Alex Gombash