It's been another strange and deadly week of weather.
And while the forces that shape the daily forecast are complex, one factor people may not always think about is cities.
If you’ve ever been in a big city during the summer, you may have felt the "urban heat island" effect. It's caused when heat gets re-radiated by pavement and buildings.
According to a new study published the week in the journal Nature Climate Change, researchers now say that large cities around the globe are more like an “urban heat archipelago,” and together they influence the flow of the jet streams circulating in the earth’s atmosphere.
And that can change weather patterns thousands of miles away.
"Thermal mountains"
The sun is the world's largest heat source, and as it warms the earth's surface, it causes the natural atmospheric circulations that move energy around the globe.
But many of largest cities in the Northern Hemisphere, like New York City, lie directly under major atmospheric circulations.
Dr. Ming Cai of Florida State University is one of the study's lead authors. He says you can think of a large city as a sort-of "thermal mountain" that interferes with these circulation systems.
"So the air that goes from West to East can 'see' these things [like New York City] from far away," says Cai, "It tries to get around this thermal mountain, a thousand miles away ... So it starts turning North and South."
The northward turn can cause warming in higher latitudes, in places like Iowa, Chicago and even Canada. The southward turn can cause cooling.
Cai says this finding explains discrepancies between real-world observations and climate model projections.
"In previous studies people haven't considered the effects of heating on the larger scale circulation. Basically they assumed larger scale circulation is business as usual, nothing changed."
But According to Cai, this new understanding of the role of cities will help improve future climate models.
"[Humans] do have a meaningful impact on nature," he says, "Even at the global scale, besides the greenhouse gases, we actually effect the heat, and the energy released creates its own kind of circulation."
Cai's co-authors include researchers from the Scripps Institution of Oceanography, U.C. San Diego, and the National Center for Atmospheric Research.
The study was funded by the U.S. Department of Energy, the National Science Foundation, and the National Oceanic and Atmospheric Association.
