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Is it always sunny in Philadelphia?
Maybe not, according to new research published in the Proceedings of the National Academy of Sciences of the United States of America. Eighteen years of daily satellite images capturing the skies above 447 medium and large cities in the United States reveal that clouds form more often above urban areas than surrounding rural landscapes, especially at night and during the summer.
Though experts have clear evidence that cities are hotter than rural areas—a phenomenon called the urban heat island effect—how this relates to local cloud patterns has been less clear, said Thuy Trang Vo, a doctoral candidate at the University of Alabama in Huntsville who led the study presented at AGU’s Fall Meeting 2022.
“We really care how urban environments impact the overall long-term trends of the atmosphere,” Vo said.
Vo and her adviser, Leiqui Hu, a professor of atmospheric and Earth sciences, took a comprehensive look at cloud coverage across seasons and climates—temperate, arid, and cold—to tease out how city size and proximity to mountains or coasts influenced cloud formation.
Forgoing physical monitoring, the researchers used more than 10 terabytes’ worth of images collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite to inspect clouds over cities housing more than 50,000 residents in the contiguous United States.
Vo plucked daily images taken at 1 p.m. and 1 a.m. local time between 2002 and 2020 and decided whether each 1-kilometer pixel had clouds or no clouds. She compared the number of cloudy pixels within the city limits to those outside the city.
A Chance of Cloud Coverage
The researchers found that the skies above cities saw 3%–6% more cloud cover in the summer than those above rural areas, regardless of size. Summer nights were also twice as cloudy as summer days. Rising heat from cities—absorbed during the day and released at night—pulls moisture up from the surrounding landscape, creating clouds, the researchers hypothesized.
Winter dampened the urban cloud effect across cities. The heightened cloud effect at night disappeared, with a more than 1% decrease in cloud coverage during winter nights compared to winter days.
Inland, mountainous, and coastal cities shared the same seasonal trends—skies were cloudier during summer nights and clearer during winter months. A lack of breeze circulation may explain why inland cities saw 5% more clouds during summer days and why coastal cities experienced an increase in nighttime summer clouds (6%) because of hotter surface temperatures retained by urban infrastructure, Vo said.
The detailed nighttime cloud trends are significant and striking, said Aaron Alexander, a doctoral candidate at the University of Wisconsin–Madison studying water resource engineering in urban landscapes who was not involved in the research. Measurements of turbulence are difficult to collect at night, when the winds are calm and temperatures are cool, using physical monitoring devices, Alexander said. This makes it tricky to study how and why severe weather phenomena such as thunderstorms, tornadoes, and hurricanes often intensify at night in some areas of the United States.
But satellite images of cloud coverage could provide a way forward in understanding the atmosphere at night. Cloud coverage is the “smoking gun” for severe weather, Alexander said.
“The nighttime atmosphere is one of the most complicated things that you can try to study in atmospheric sciences,” he said. “The fact that we now have this extra data point is very exciting.”
In future studies, the team hopes that cloud maps overlaid on maps of air pollution will illuminate how aerosols affect clouds, Vo said. For now, she said she hopes this work can inform urban infrastructure design to improve the ways cities retain heat and handle extreme precipitation.
—Elissa Welle (@ElissaWelle), Science Writer