Early Friday morning, Delhi's air quality fell to 'severe' as weather conditions made the environment conducive to trapping pollutants.
The air quality index (AQI) hit 424 as pollutants remained trapped due to a cold wave and low wind speeds which made the air heavier.
The weather impacted flight operations and schools were ordered shut in the National Capital Region.
Here are more details.
Delhi's AQI fell to 424 from 362 on Thursday
At 7:30 am on Friday, Delhi's AQI docked at 424, falling in the 'severe' category, according to the Central Pollution Control Board (CPCB). The AQI worsened from Thursday, when it stood at 362 at 4 pm.
An AQI between 201 and 300 is considered 'poor', 301-400 'very poor' and 401-500 'severe', while the AQI above 500 falls in the 'severe plus' category.
Dense fog blocking sunlight; causing fall in temperature: Officials
On Thursday, Delhi recorded minimum temperatures of 5.2°C with maximum temperatures touching 15°C, seven notches below normal, the India Meteorological Department (IMD) said.
Officials said the steep fall in temperatures was due to the dense fog, which prevented sunlight from heating the ground.
Similar conditions are expected to prevail on Friday, with maximum and minimum temperatures forecasted at 16°C and 6°C respectively.
All Noida, Greater Noida and Ghaziabad schools shut
Amid the prevailing cold wave, all schools across Noida, Greater Noida and Ghaziabad have been ordered shut on Friday. An FIR was also filed against 13 schools in Ghaziabad which violated the state government's order and remained open on Thursday.
Due to bad weather, flight operations affected: Delhi airport
Meanwhile, flight operations remained disrupted on Friday morning.
The Delhi Airport's official Twitter handle announced that as of 7 am, bad weather conditions continued to impact flight operations.
While take-offs and landings continue, some flights are impacted due to crew operational limitations, the notification read.
Passengers are advised to check their flight status before leaving for the airport.
Not just Delhi; cold wave gripped several Northern states
In Himachal Pradesh, while most regions witnessed dry and overcast conditions, Keylong recorded the lowest temperature at -12°C on Thursday.
In Haryana, Karnal recorded a low of 4.6°C, Ambala 7.4°C, while Narnaul recorded temperatures of 3.5°C. Punjab's Patiala recorded a low of 5.5°C.
Union Territory Chandigarh recorded a minimum of 9.9°C.
Minimum temperatures reached -2.3°C in Srinagar, while Drass recorded a low of -14.4°C.