Thick grey haze choked Delhi for the third straight day on Tuesday, with the pollution levels remaining alarmingly high at 488 in the 'severe plus' category.

Calm winds and dropping temperatures made the dispersion of pollutants difficult. Cold air traps dust and smoke from stubble burning in neighbouring areas of Delhi.

The city recorded its coldest night of the season so far on Monday with the mercury dropping to 12.3 degrees Celsius from 16.2 degrees Celsius the night before, according to the India Meteorological Department (IMD).

Delhi's Air Quality Index (AQI) was recorded at 488 at 9 am, according to Central Pollution Control Board (CPCB) data.

Out of the 32 monitoring stations in the capital, 31 recorded AQI levels exceeding 480. Two stations, ”Alipur and Sonia Vihar, maxed out at 500.

On Monday, Delhi recorded its second-worst air quality in six years with the AQI touching 494.

Delhi's air quality first entered the 'severe plus' category on Sunday, prompting the implementation of tighter pollution control measures under Stage IV of the Graded Response Action Plan (GRAP) the next morning.

These measures include a ban on the entry of trucks except for those carrying essential items or using clean fuel (LNG/CNG/BS-VI diesel/electric), a ban on construction and demolition activities and school closures.

First implemented in 2017, the GRAP is a set of anti-air pollution measures followed in the capital and its vicinity.

Based on the severity of the situation, it classifies the air quality in the Delhi-NCR under four different stages: Stage 1 - 'poor' (AQI 201-300), Stage 2 - 'very poor' (AQI 301-400), Stage 3 - 'severe' (AQI 401-450) and Stage 4 - 'severe plus' (AQI above 450).

Vice President of Meteorology and Climate Change, Skymet Weather, Mahesh Palawat said the combination of dense fog in Punjab, Haryana and Delhi coupled with slow northwest winds is trapping pollutants in the air.

"The dropping temperatures are causing cold winds to push pollution closer to the surface. Without an increase in wind speed, a significant improvement is unlikely over the next two to three days. However, rain is expected later this week and it could help reduce pollution levels," he said.

Fog reduced visibility to 400 metres on Tuesday morning and it is likely to persist during the day, the IMD said.

The humidity levels were recorded at 89 per cent at 8.30 am. The maximum temperature is expected to settle at 24 degrees Celsius, it said.

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