Yesterday, on December 19, many parts of North India including Punjab Haryana and parts of Uttar Pradesh witnessed dense fog. Today on January 20, the density and spread of fog increased. It was seen from central Pakistan to Uttar Pradesh across Punjab, Haryana, and Delhi. Visibility at Palam airport dropped to 50 meters, Lucknow and Prayagraj airport also witnessed almost Nil visibility.
Moisture is available in the atmosphere and winds are light at lower tropospheric levels over Indo-Gangetic plain. Due to clear sky and overnight cooling, the surface of earth down rapidly and air near the Earth's surface also become colder and heavier. The moisture available in the air condense and overcome fog. Visibility dropped over Delhi due to the formation of smog. When pollutants such as dust particles, smoke particles and particles of harmful gases get mixed with water vapor it becomes smog.
Temperatures will remain low, and winds will also so be light for at least next 2 days. Therefore, we do not expect any significant relief from poor visibility, dense fog and very poor air quality. Fog will start lifting and get thinner as the day progresses. Visibility may start dropping once again from late evening or early night. Rail, road, and Air Traffic may get impacted due to fog and poor visibility.