By Lauren Lipuma

Scientists have observed discrete holes in widespread fog directly above cities in India, other regions of Asia and Europe. The fog holes increase in size as city populations increase, and the researchers suspect the urban heat island effect is likely to blame for the striking phenomenon.

Air pollution can increase fog formation, and widespread fog is known to affect several densely-populated areas around the world. In late fall and winter, areas like the North China Plain, California’s Central Valley, the Po Valley in Europe and the Indo-Gangetic Plain in India experience persistent fog that stretches for hundreds of kilometers and can impact transportation, air quality and public health. The dense and polluted fog over India is a major environmental concern that causes massive delays and cancellations to flights and trains and even automobile accidents every year during the winter months.

Researchers in India were using satellite imagery to study the connection between fog and air pollution when they noticed something strange – holes in widespread fog that appeared directly above densely populated cities in the Indo-Gangetic Plain, including Delhi, a city of 19 million people.

In a new study published in Geophysical Research Letters, a journal of the American Geophysical Union, scientists analyzed satellite images of the fog holes from 2000 to 2016. They compared the hole size to each city’s surface area and population, and found the holes were bigger over larger and more densely populated urban areas.

This image captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite on December 7, 2016, shows widespread fog blanketing northern India, with fog holes over major cities clearly visible in the top left corner of the image. 
Credit: NASA.

A close up of fog holes over cities in northern India from the above image, with several major cities (and their associated fog holes) labelled in red.
Credit: NASA.

“We found a very strong correlation between these two variables: the population of the cities and the fog hole area, and we were surprised to see such a strong correlation. In comparison to multiple cities globally, we found the largest and most frequent holes in fog over Delhi,” said Ritesh Gautam, a former professor at the Indian Institute of Technology Bombay who is now a scientist at Environmental Defense Fund in Washington, D.C., and lead author of the new study. “Our hypothesis is that the fog is dissipating a lot faster over these urban heat islands compared to the surrounding area, and in some events the fog is not even forming over the cities.”

The study authors suspect the urban heat island effect is responsible for creating the fog holes. Over the 17 years of satellite data the researchers studied, they saw fog increasing over the Indo-Gangetic Plain, but observed a 50 percent reduction in fog, on average, over Delhi compared to its rural surrounding areas, according to Gautam.

Gautam suspects two processes contribute to fog hole occurrence over large cities. Cities are generally warmer than their surroundings, because city surfaces and building materials like concrete significantly absorb more heat than their rural surroundings, a phenomenon known as the urban heat island effect.

These urban heat islands could create fog holes by impacting the lowest part of the atmosphere, known as the boundary layer, which needs to be stable and enriched with moist air for fog to form. Warmer urban areas could also reduce the relative humidity, leaving less moist air available to form fog.  

In addition, plants release water vapor to the atmosphere through a process called evapotranspiration. In cities, where there is less green cover than the surrounding country, the relative humidity is lower, which could account for the holes over populated areas.

The presence of fog holes adds a new dimension to the issue of human influence on weather and low-level cloudiness, according to the study’s authors. Aerosols from pollution act as seeds on which clouds form, which increases fog formation, but the new study shows urban heating decreases fog formation, having the opposite effect.

“We have to pay attention not only to air pollution but also to urban heating, and try to derive the relative importance of these two factors in fog dynamics and future changes in fog frequency,” Gautam said.

— Lauren Lipuma is a senior public information specialist and science writer at AGU. Follow her on twitter at @tenacious_she.