The presence of a Western Disturbance (WD) over the Western Himalayan region is making the conditions conducive for fog in northwest and Indo-Gangetic plains.
The weather system (WD) which is basically a low pressure area originates in the Mediterranean Sea and adjoining Atlantic Ocean and travels east or northeastwards to reach the Himalayas, creating a high pressure gradient between the hills and plains in North India.
The high pressure zone prevails in the plains while the low pressure zone settles over the hills during the winters. This pressure gradient leads to change in wind conditions, thus moist winds from the Bay of Bengal begin to blow over regions that witnesses low night temperatures (another factor for fog formation during this time of the year).
Western Disturbance also works when southeasterly winds are weak and unable to reach the northwest plains. In this condition the WD slows down the speed of northerly winds which is the case in North India right now. For instance, fog in Delhi that was witnessed today due to slow wind speed of around 5 kmph, ideal for fog formation.
More heating in the plains during the day in winters leads to a shift in the pressure gradient, whereby a low pressure area develops over the plains while the hills turn into a high pressure zone. This high pressure gradient changes the wind speed that finally leads to a gradual reduction in fog as the day progresses.
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