‘Advective’ and ‘Radiation’, two different forms of fog in India generally form in the sub tropics, around latitude 25⁰. The former category of fog forms in the coastal areas due to constant moisture feed from the water bodies and the latter forms on a humid night with clear skies, after the passage of a Western Disturbance.
Below is an explanation of how and why fog in India forms in different regions:
North India– Dense fog in Delhi and North India (from December till the end of January) is the most common feature of the winter season which leads to massive delays and cancellations of flights and trains. The reason for fog in North India is entirely because of a weather system called Western Disturbance. A Western Disturbance affecting the hills of Jammu and Kashmir may sometimes lead to a sudden reversal of winds from the normal dry and cool northwesterly to humid easterlies or southeasterlies which could cause widespread fog in the plains of North India. Moreover, light variable or calm winds (speed below 5kmph) do not allow fog to lift and lead to further continuation of fog for a longer duration.
Fog in Delhi and NCR could also appear after a spell of widespread rain due to a Western Disturbance as enough moisture content trapped in the atmosphere leads to fog formation.
Besides fog in cities of Punjab and Haryana, fog in Delhi is the worst every winter.
East and Northeast India– Fog in East India has also been very thick and dense causing major flight and rail delays. However, the reason for fog formation in this part of the country is primarily due to its proximity to water bodies (River Ganga) and slow wind speed.
In the hilly areas of the Northeast, characteristics of fog are completely different from what they are in the plains of East India. The terrain of Northeast India is a mix of everything, from hills and valleys to mountains, which is conducive for trapping fog for longer durations. Northeast is well known for dense fog throughout the winter season due to persistent flow of moist air from River Brahmaputra.
Lucknow, Allahabad in Uttar Pradesh, Patna in Bihar, Kolkata in the Indo Gangetic plains and Jorhat in Northeast India are some of the places that witness dense to very dense fog during winters.
Central India – Dry Central India is generally escapes the wrath of fog in the winter season. However, places closer to the sub-tropics (at around latitude 23⁰N) like Jabalpur, Indore and Nagpur do witness fog sometimes. Changing weather conditions in North India leads to rare occasions of fog in Central India.
Quite surprisingly, region of tropical climate, Gujarat observes widespread fog not just in the winter season but also during summers. This is again more common in coastal stations of Porbandar, Dwarka and Jamnagar.
South India – Fog in South India could sound quite surprising to those who thought fog was a common feature only in the northern parts of India. In spite of being surrounded by water bodies, fog in South India isn’t a very prominent feature and is generally location specific. Bangalore for example is known for dense fog and smog in the winter months, largely because of the high air pollution levels and adequate moisture content in the air.
