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In India, a Mystery of Cities and Holes in the Clouds

The urban heat island might be responsible for a strange weather pattern that appeared this week.

Looking down on India Wednesday you might think somebody had dropped several huge pianos on the country, leaving the clouds riddled with holes located precisely above cities.

NASA

What was going on? Well, even the folks at NASA aren’t entirely sure, but they hazard a guess: the urban-heat island. The agency’s Kathryn Hansen writes:

The holes appear similar to hole-punch clouds, which occur when super-cooled liquid clouds are disturbed, usually by the particles in airplane exhaust. The exhaust triggers a cascade of freezing: ice particles bump into liquid drops, and then larger ice particles literally fall out of the sky and leave behind a hole.

But [NASA meteorologist Steve] Lang and colleagues checked other lines of evidence—different MODIS channels, local reports, and soundings—and think that these are water clouds (essentially fog). It could be that the fog was burning off faster over the cities, which tend to be warmer than their surroundings.

“Most likely, the urban heat island is added to the effects of the Sun as it burns off the fog,” Lang said. “It could also be that the fog was thinner over the cities to begin with due to the heat island effect.”

So credit the heat island for probably adding another quirk to nature, in addition to reducing rainfall and acting like steroids for trees. Here’s another view of the perforations, as well as the grayish smog that always seems to shroud the region:

NASA

About the Author

  • John Metcalfe
    John Metcalfe is CityLab’s Bay Area bureau chief, based in Oakland. His coverage focuses on climate change and the science of cities.