Megacities Like Paris and London Can Produce Their Own Clouds

Scientists have long known that bustling metropolises like New York City, Paris and London are considerably warmer than their surrounding countryside. This phenomenon, known as the urban heat island, stems from the vast amount of energy produced by cities, which house a disproportionately high number of humans in an increasingly cramped space.

But a new study published in NPJ Climate and Atmospheric Science is one of the first to highlight a separate yet related aspect of urban environments. As researchers led by Natalie Theeuwes, a meteorologist at the University of Reading, report, megacities’ size and structure also appear to enable them to create their own clouds. In fact, Tom Bawden writes for I News, the team’s findings suggest that megacities tend to have up to 10 percent more cloud cover than the rural regions around them.

According to Science magazine’s Paul Voosen, this conclusion is surprising given cities’ relative dryness, as precipitated by a lack of vegetation. Logically, such conditions should lead to less water evaporation and subsequent cloud formation, but the team’s ground-based London observations suggest otherwise, pointing toward heat production as a key driver of cities’ cloud cover.

In simple terms, clouds are created when moisture-filled warm air rises up into the atmosphere, cooling down and condensing to form the puffy, cotton candy-shaped structures seen dotting the sky.

Thanks to vegetation, rural air tends to contain more moisture than urban air. But as Bawden explains, the scientists found that the level of heat produced by megacities is more than enough to offset their air’s lower moisture content. Thermal updrafts exacerbated by urban energy carry high quantities of air upward; while this air holds less moisture than rural air, the sheer scale is capable of generating larger clouds that cover cities for longer periods of time than rural regions. The result, according to the researchers, is persistent urban cloud cover in the afternoons and evenings.

“While the rural surroundings start to cool and there is no more energy for these updrafts, clouds die out,” Theeuwes tells Atlas Obscura’s Sabrina Imbler. “But over the city there is still heat and energy available at the surface to maintain these clouds.”

Speaking with I News’ Bawden, Theeuwes adds that the research “reveal[s] the increasing impact cities are having on their own mini-environments.” Although this latest study focused on satellite observation of London and Paris, as well as on-the-ground data collected in and around the English capital, Theeuwes says that numerous urban centers—including Sacramento, Mexico City, Basel, Cairo and Helsinki—offer ample evidence of the link between cloudy days and excess heat.

As the researchers conclude in the paper, “Urban areas are seen to directly affect weather phenomena besides temperature, impacting the city’s inhabitants.”

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's senior associate digital editor, history.