Dense fog, thick enough at some places to cut visibility to below 50m, is likely to engulf swathes of northwestern states over the next three to four days, the India Meteorological Department (IMD) said on Wednesday, urging residents to be alert for possible impacts.

The IMD said “dense to very dense fog” is “very likely” during the night or morning hours over many parts of Chandigarh, Delhi, Haryana and Punjab between Wednesday night and Friday morning and some parts during the subsequent three days.

Dense fog can reduce visibility to between 50m and 200m, while very dense fog
can reduce visibility to below 50m, under weather terminology. An event classified as “very likely” has a 50 per cent to 75 per cent chance of occurring.

The IMD said dense to very dense fog is also very likely to occur in some parts of Uttar Pradesh over the same period. It also said dense fog is very likely over isolated pockets of northern Rajasthan and Odisha on December 28 and across northeastern states during the morning hours of December 28-31.

The fog may affect airports, highways and railway routes in some areas, the weather agency said, cautioning residents to be alert about “difficult driving conditions with slower journey times”, and warning that without precautionary measures,
road traffic collisions may occur.

Dense fog contains tiny particulate matter and other pollutants and exposure can increase the risk of wheezing, coughing and shortness of breath, particularly in people with underlying or vulnerable to respiratory disorders. Long periods of exposure to dense fog may exacerbate respiratory symptoms in people with asthma or other lung disorders.

The IMD’s advisory said some power lines could trip in areas under very dense fog conditions, and has urged power companies to keep maintenance teams prepared. The advisory has urged residents with travel plans in fog-affected regions to check with airlines or railways about aircraft or train schedules.