Explanation:

• There is a connection between fog conditions in the Indo-Gangetic Plains (IGP) and anomalies in the movement of air in the Arctic and Eurasian regions.
• There is a ‘footprint’ of the Arctic Oscillation (AO) and Eurasian (EU) circulations on the anti-cyclonic circulations — regions of high-pressure areas associated with sinking air — over the north Indian plains.
• During the winter season, when temperatures are cold, these circulations push air towards the surface and trap water vapour in its place, producing foggy weather. The water vapour for this phenomenon could be present in local atmosphere at the time or sourced from nearby oceanic regions by the circulation. 
• High pressure systems in the Arctic Circle during the negative phase of AO phenomenon increases the occurrence of fog events in the IGP.
• During the positive phase of the AO, colder temperatures are confined around the Arctic region and it’s well separated from the tropical regions keeping them warm. But during its negative phase, the Arctic circulation gets disrupted and the cold air can move out of the Arctic region and affect weather in the mid and lower latitudes of North America, Europe and Asia. This can cause frigid winters in these regions.

• Another semi-permanent climatic phenomenon that the scientists say affects the fog conditions in the IGP is a large anti cyclonic system over the Siberian region which forms during the winter season.
• The positive phase of this system, also known as the Siberian High (SH), is also associated with foggy weather, but to a lesser extent. Positive SH sends cold winds to the IGP even during the positive AO phase.
• When both the negative AO and positive SH are aligned with each other they bring cold air towards the south and create the most favourable conditions for the formation of fog in the IGP. 
• The current winter season in North India, especially Delhi, has witnessed less than normal fog events.
• Anti cyclones and fog conditions when combined with the increasing air pollution levels in cities like Delhi can create deadly smog, which is detrimental to the health of the people. The three years have witnessed smog events in the capital.
• In November last year, the situation was so critical that Delhi came close to banning all private vehicles in the city, but was later saved by timely rainfall which brought down the pollution levels.
• In the first week of November 2017, a north westerly wind had brought in dust along with smoke from stubble burning from north-west India and Pakistan inducing intense smog.
• In 2016, Delhi had seen high levels of pollution around Diwali and the worst smog in 17 years. An anti-cyclonic system which had drastically reduced wind speeds in Delhi had worsened air quality during this period.
• But anti cyclones and fog conditions are not the only Indian weather phenomena that are affected by global climatic cycles. 

• The AO (Arctic Oscillation) and the El Nino Southern Oscillation (ENSO) are two major global phenomena that influence Western Disturbances (WDs), also known as the Indian winter monsoon.