Dense to very dense fog usually occurs over northern pains including Delhi NCR during the month of December and January. December 2020 was an exception when Palam Airport witnessed only one hour of very dense fog when visibility reduced to zero. The intensity of fog starts reducing in the month of February. Dense to very dense fog is common over Delhi, particularly over Indira Gandhi International Airport until the first 10 days of February. Indira Gandhi International Airport of Delhi witnesses on average 12 hours of dense fog in the entire month of February. But this year until February 19th, Palam Airport has already witnessed 13 hours of dense fog.
Day temperatures are above normal over most of the plains of North India including Delhi, since last many days. Minimum temperatures for most of the plains of North India including Delhi are also near normal. The reason for this unusual spell of dense fog over Delhi and North India is light winds. The successive Western disturbances in the month of February have not given moderate to heavy rain over Delhi. But these were instrumental in changing wind direction frequently. Light and slow winds over the plains of North India are not letting the pollution particles to disperse. Pollutants are accumulating leading to a reduction in visibility. Pollution in combination with fog has resulted in such poor visibility over most of the plains of North India. We can say the phenomena which is being witnessed over most of the plains of North India now is a combination of pollution and fall and can be termed as smog.