What is fog hole seen above Delhi?

Winter in North India is known for its chilling cold and thick fog, especially in cities like Delhi. This fog often disrupts daily life and transportation. A recent study by researchers at the Indian Institute of Technology Bombay has uncovered a new player influencing the notorious winter fog in North India. The culprit? Urban heating.

Urban heat creates large ‘fog holes’

The study found that increased land surface temperatures in cities disperse the fog cover faster than in rural areas. This creates ‘fog holes’, or open patches in the fog, that can be as large as Delhi when viewed from space.

Fog is a type of low-lying cloud that forms from moisture condensation. In North India, the combination of moisture from agricultural land, water bodies, and calm winds leads to dense fog formation. While fog can sometimes be a nuisance, it also plays a crucial role in the region’s ecology and aids the growth of certain fruit trees.

The journal Geophysical Research Letters published a study revealing a high correlation between the geographical extent of the 'fog hole' and a city's population. The larger the city population, the larger the fog hole is.

The researchers used data from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) satellite over 17 years (2000-2016). They observed ‘fog holes’ occurring over urban areas of the Indo Gangetic plains, with the most prominent one over Delhi.

Larger cities show bigger fog holes

The researchers observed a direct correlation between the size of the fog hole and a city's population. Larger cities, such as Delhi, exhibit more prominent fog holes.

Ritesh Gautam, formerly a professor at IIT Bombay who led the study, observed a strong correlation between the fog hole area and populations in 13 global cities representing USA, Europe, and Asia.

He further notes that among these cities, Delhi's urban heat appears to have the strongest impact on the suppressed fog formation when compared globally.

Urbanization, it seems, is altering the fog dynamics. Unlike rural areas with cooler temperatures and more vegetation, cities experience urban heating, especially during winter months. The lack of green cover and higher temperatures create lower relative humidity within cities, leading to distinct differences in fog formation.

How does heat affect fog formation?

Cities are generally warmer than rural areas due to intense urbanisation, especially during winter months. This, along with a lack of vegetation cover within city premises, leads to low relative humidity inside cities compared to rural areas. This can cause sharp differences in fog formation across urban-rural landscapes.

The study notes that while aerosols in the air can increase or intensify fog development, the effect of urban heating seems to be more dominant, effectively causing fog holes.

These findings could significantly impact fog prediction. A better understanding of fog processes and the specific impact of urban heating on fog could improve fog forecasting capability.

The study indicates that aerosols, or fine particles from urban pollutants, can enhance fog formation. However, the influence of urban heating is more significant, leading to the creation of ‘fog holes’.

Prof. Gautam adds, "This finding could considerably impact fog prediction. The study results move towards a better understanding of fog processes and highlight the specific impact of urban heating on fog. If we developed a sophisticated prediction system that accounted for the impacts of both air pollution and urbanisation, we could improve our fog forecasting capability."

The research provides clear evidence of how urban heating contributes to the formation of fog holes over cities. As Prof. Gautam highlights, fog is a regional concern, not just a local one. Given the ecological importance of fog and its significant impact on life in the northern plains, there is a need for comprehensive research into fog formation processes and the effects of air pollution and urbanization on fog, not only in Delhi’s urban areas but also in other parts of North India.

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