The India Meteorological Department on Monday issued an orange alert for dense to very dense fog conditions in Delhi and its adjoining areas.

An orange alert requires authorities to “be prepared” in view of an extreme weather event.

While a red alert means that the administration is expected to “take action”, a yellow alert means that authorities should “be updated” on the situation.

On Monday, several trains were delayed or cancelled due to the fog in the national capital, India Today reported. The visibility at the Indira Gandhi International Airport dropped to 50 metres.

“Low Visibility Procedures are still in progress at Delhi Airport,” the airport operator said at 9.05 am. “However, there has been no impact on flight operations. Passengers are requested to contact the airline concerned for updated flight information.”

The weather department on Sunday said that dense to very dense fog conditions are likely to continue in northwestern India for the next two to three days.

On Monday, the daily average air quality index in Delhi stood at 262 at 11.05 am, according to data from the Central Pollution Control Board.

An index value between 0 and 50 indicates “good” air quality, between 51 and 100 indicates “satisfactory” air quality and between 101 and 200 indicates “moderate” air quality.

As the index value increases further, air quality deteriorates. A value of 201 and 300 means “poor” air quality while between 301 and 400 indicates “very poor” air.

A value between 401 and 450 indicates “severe” air pollution and anything above the 450-mark is termed “severe plus”. At these levels, healthy people can experience respiratory illnesses from prolonged exposure to air pollution.

On Sunday, the Commission for Air Quality Management revoked emergency measures under stage 3 of the Graded Response Action Plan in the National Capital Region as the region’s air quality improved.

The air quality panel noted in its order that Delhi’s air quality index at 4 pm on Sunday was recorded at 278. This was 72 points below the 350-mark, at which grade 3 of the incremental anti-pollution measures have to be implemented.

Measures under stage 2 of the graded action plan, however, remained in force. This means that civic bodies have to carry out mechanical or vacuum sweeping of roads daily and strictly enforce dust control measures.

The air quality panel had reimposed emergency measures under stage 3 on January 9 as the air quality in Delhi plummeted to the “very poor” category.