21 December 2022

Table of Content

As fog comes to Delhi, a look at the phenomenon, and what causes it

GS-3: Conservation, Environmental Pollution and Degradation, Environmental Impact Assessment.

Since the past few days, dense fog has covered north-western India including Delhi, Punjab, Haryana and parts of Uttar Pradesh, and Rajasthan. It keeps visibility poor in the hours before and after daybreak.

What is fog and how does it form?

• What? Fog is a cloud of small water droplets suspended in the air near to the ground.
• How? When water vapor condensed, during the condensation, molecules of water vapor combine to make tiny  little water droplets that suspend in the air.
• When? Whenever there is a temperature difference between the ground and the air, the fog is formed.
• Factors responsible: 1) Moisture in the air and 2) Fall in temperature.

• The intensity of fog depends on factors like humidity, wind, and temperature, which allows higher spatial variability in formation of fog.

• For example, Areas near water bodies may have denser fog due to higher humidity.

• Why? During the nights, land surface cools down and consequently air closer to the surface also cools down. Since, cooler air cannot hold as much moisture as warm air, the water vapour in the air condenses to form fog.

Fall in temperature and Western Disturbances

• Western Disturbances (WD) are the extra-tropical storm originating over Mediterranean Sea or Caspian sea that bring moisture bearing winds to northwest India, which results in increased moisture levels in the region.
• In “Western Disturbance”, the term “Western” refers that the disturbance travels from the western to the eastern direction and the term “disturbance” refers a low pressure area. 
• WD are strongest in January-February and weakest during monsoon months in India.
• It moves across Iran, Iraq, Afghanistan, and Pakistan before arriving in India.
• It causes rainfall, snowfall and fog in northern India. Thus, WD causes most pre-monsoon or winter rainfall in North-west India. It accounts for 5-10% of India’s annual rainfall.

• WD is also associated with Cloudy skies, warmer night temperatures, and unexpected rain.

• Strong WD (Rain in the winter) helps Rabi crops such as Wheat, barley, mustard, gram, lentil, etc. and also helps residents, farmers, and governments to tackle water scarcity.

• On the contrary, weak western disturbances causes crop failure and water shortages in North India.

Types of fog

• Radiation fog: Categorized by SAFAR forecasting system as a localized fog, formed due to calm winds and 

• Fog over Delhi is Radiation type of fog.

• Advection fog: It mainly occurs in the region where warm tropical air meets cooler ocean water. This sea fog is transported over coastal land areas by the wind flowing in the right direction. 

• Principle: When warm moist air passes over a cool surface, it causes water vapour to condense.
• Advection fog is larger in scale both in terms of the area covered and duration. 

• Valley fog: It mainly occurs in the mountainous region where mountain prevent dense air from escaping, and the fog is trapped in the bowl of the valley.
• Freezing fog: It mainly occurs on the cloud-covered mountaintops and not applicable to the Indo Gangetic Plain. 

• It occurs due to freezing liquid droplets on solid surfaces. 

Link between pollution levels and fog

• Local wind speed falls with the decline in the temperature. In addition, the inversion layer also comes down and vertical mixing reduces, too. 
• Consequently, it lead to fog formation and in result, particulate matter hangs on the lower boundary levels, which increases pollution.

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