NEW DELHI: The national capital recorded the coldest morning with a minimum temperature of 3.3 degrees Celsius, four notches below the seasonal average, the India Meteorological Department (IMD) said.

Along with the bone-chilling cold, Delhi-NCR was wrapped in a blanket of fog that led to delays in 18 trains and several flights due to poor visibility.

Due to the dense fog it had lowest visibility at zero in Safdarjung and Palam at 2 a.m.

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

The 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 Indian Railways, a total of 18 trains were running late by hours due to fog and cold wave.

The traffic on the roads was extremely slow as fog reduced the visibility.

Meanwhile, several areas in Delhi were experiencing 'severe' air quality, prompting the weather office to issue a red alert due to the prevailing cold and fog conditions.

The IMD forecast indicated dense fog in Delhi until Tuesday.