Urban heat island burns hole in fog over Delhi

Every winter, the whole of north India is covered by dense fog. But a phenomenon called urban heat island is burning holes in this grey shroud over New Delhi and other cities on the Indo-Gangetic Plain, says a new study published this week.

The urban heat island effect is so strong in Delhi, the largest city in the region, that it saw 50 per cent less fog than surrounding areas, according to an analysis of satellite data between 2000 and 2016 by Ritesh Gautam, a former professor at IIT Bombay who is now with the Environmental Defense Fund in the US, and Manoj Singh of the University of Petroleum and Energy Studies in Dehradun.

Urban heat island refers to the higher temperature seen in a city compared with adjacent rural areas because of intense concrete development and reduced green cover.

Warmer temperatures help fog dissipate. In Delhi, the heat island effect also appears to be suppressing the very formation of fog, said Gautam. Scientists found that while areas outside Delhi have seen a 20 per cent increase in fog in the period 2012-2016 compared with 2000-2004, Delhi itself did not see an increase.

Another surprise, said Gautam, was to find less fog in Delhi even at night though temperatures are cooler. There were also differences in fog duration—in one event, fog in Delhi dissipated three hours before surrounding rural areas.

Globally, Delhi saw larger and more frequent fog holes in this period than fog-prone cities in the Po valley in Italy, the North China Plain (including Beijing), and California’s central valley, the study found. The analysis found a correlation between the size of the urban population and that of the fog hole.

Population size has been shown to be related to the intensity of urban heat islands since they are an indicator of urban growth, noted Gautam.

Fog holes also appear frequently over Lahore, Amritsar, Jalandhar, Ludhiana and Patiala in the Indo-Gangetic Plain, where fog was found to be 17-36 per cent less than their surrounding areas.

Interestingly, long-term declines in fog in Europe and the US had long thought to be related to declines in air pollution since pollution particles known as aerosols amplify fog. However, recent studies have begun to point to urban expansion as a factor too.

The study highlights this paradox of fog formation. High levels of air pollution may help amplify fog over north India—but urban development may suppress or dissipate it. “The degree to which one cancels the effect of another is hard to say,” said Gautam. “The next step in our work is to tease out the relative importance of these two factors.”

The study, the first to show direct impact and evidence of urban hot spots on fog globally and specifically over Delhi, was published in Geophysical Research Letters.