New Delhi, Dec 25 (IANS): It was a cold foggy Tuesday morning in the national capital with visibility dropping to 50 metres affecting flight schedule and causing train cancellations.

The air quality in Delhi-NCR remained 'severe' for the fourth consecutive day as high humidity, low wind speed and drop in mercury prevented the dispersal of particulate matters.

According to SAFAR, the Air Quality Index (AQI) of the national capital was 423 -- in the hazardous zone -- as the minimum temperature was recorded at 5.6 degrees Celsius, two notches below the season's average.

The India Meteorological Department (IMD) has predicted shallow fog for the day with the maximum temperature likely to hover around 21 degrees Celsius.

At 8.30 a.m., the humidity was 100 per cent; visibility dropped to 50 metres at Palam with a spate of flight delays at the Delhi airport.

All departures at the the Indira Gandhi International Airport (IGI) were suspended for over an hour as dense fog delayed 45 flights.

"Due to adverse weather conditions (fog) at Delhi, few of our flights have been affected," the Jet Airways tweeted.

"The overall air quality in Delhi is severe," System of Air Quality and Weather Forecasting And Research (SAFAR) said in its daily pollution analysis.

"There is a significant fall in temperature which has brought down the boundary layer significantly with the huge amount of moisture beneath," SAFAR said.

At the IGI Airport (T3) PM2.5 was recorded at 434. Many areas in the capital recorded Particulate Matter (PM10) levels way beyond the safe standard.

Experts advised against all physical activities outdoors and prescribed the use of N-95 or P-100 respirators when stepping out.

Among trains originating from here at least four were cancelled, including the Gomti express to Lucknow.