New Delhi, Jan 3 (IANS): As the cold wave has tightened its grip on Delhi, residents of the national capital woke up to a freezing morning on Wednesday with the minimum temperature recorded at 7.3 degrees Celsius -- the seasonal average, according to India Meteorological Department.

The IMD has also predicted shallow fog and the maximum temperature is expected to hover around 18 degree Celsius.

On Wednesday, Palam recorded the lowest visibility at 500 m at 7.30 a.m.

However, at around 9 a.m. the visibility at Palam improved to 700 m, while Safdarjung recorded 500 m.

The IMD classifies fog intensity into four types: shallow, moderate, dense and very dense fog. T

he visibility ranges from 999 m to 500 m; 499 m to 200 m; 199 m to 50 m; and less than 50 m, respectively.

According to the Indian Railways, at least 26 trains were running late by hours due to fog.

The Chennai-New Delhi Duronto Express, Amritsar-Nanded Express and Ajmer-Katra Express were running behind schedule by more than six hours.

Meanwhile, theair quality continued to remain under the 'very poor' category at several stations across the city.

At the Anand Vihar area, PM2.5 levels remained into the 'very poor' category standing at 301 and PM10 reached 166 or ‘moderate’, according to the Central Pollution Control Board (CPCB).

The Air Quality Index (AQI) between zero and 50 is considered 'good'; 51 and 100 'satisfactory'; 101 and 200 'moderate'; 201 and 300 'poor'; 301 and 400 'very poor'; and 401 and 500 'severe'.

The Indira Gandhi InternationalAirport T3 also witnessed PM2.5 levels at 321, the ‘very poor’ category while the PM10 was at 221, falling under the 'poor' category, respectively.