Indian researcher Ritesh Gautam has solved a mystery that was puzzling NASA scientists: Why can satellites see clearly defined holes in clouds, some several miles in diameter? The mystery started in 2016, when NASA’s Earth-observing satellite Terra discovered some of those strange holes in giant, very thick fog cloud blankets over India.
The probable cause: cities. That’s something that NASA’s Goddard Space Flight Center’s research meteorologist Steve Lang and other people suspected since they saw Terra’s photos. After all, the holes always appeared on top of some urban centers (although not over others). Lang claims that he thought there was a connection to cities but he could only speculate on the origin because he didn’t have the information he needed: “[I]t could be due to either additional aerosols, urban heat islands, or both, having an effect on the clouds.”
Now, a new study published earlier this year by Gautam in Geophysical Research Letters, has announced the definitive answer to the mystery. The good news: It’s not because of aerosol (aka pollution) particles suspended over cities. The bad news: It is the heat cities produce.
More to the point, it’s the “heat island effect,” the tendency of cities to be hotter than surrounding areas–and an effect that can both alter weather patterns and exacerbate climate change in urban areas. Gautam’s research demonstrated that “[o]n the one hand, you have air pollution likely amplifying fog” and on the other, “urban heating is helping fog dissipate faster.” So aerosol particles help fog form. But then heat from cars, asphalt, and other urban fixtures rise up, push the fog apart, and open a hole in the sky.
According to Gautman, who studied satellite images from 2000 to 2016, there’s a high correlation between the presence of fog holes and the population density in cities situated on fog-prone areas. The reason why this effect has been observed in Indian cities like Delhi for the first time is that the density of traffic and temperature-controlled buildings has increased dramatically in recent years.
The heat island effect has a clear economic impact on cities around the world. As my colleague Kelsey Campbell-Dollaghan wrote last year, in “an analysis of the 1,692 largest cities in the world . . . the hardest-hit cities could lose almost 11% of their GDP by 2100 under the most extreme scenario, with average losses at about 5.6%” as a result of the heat island effect. In that light, fog holes can be seen as a stark visualization of a very hot, expensive problem.
But there are solutions: Urban planners, architects, and city authorities can implement measures to diminish or counteract the effect. Using materials like cool roofs and cool asphalt can help. Energy-efficient buildings will have an impact too. Then there are solutions as simple as planting more trees and vegetation. According to NASA, greenery-dense cities–including roofs with gardens and buildings covered with plants–can lower their temperature by as much as 3.4º F in the summer and 2.7º F in the winter. The added bonus: Cities will look prettier and trees and vegetation have a demonstrated positive psychological impact on humans.