Since Saturday, Delhi's air quality index has dramatically dropped by 60-80 points and is now inching towards the 'very poor' zone, when children are advised not to spend time outdoors. Weather condition too have played a role in the appearance of smog, Met officials said.

"But such a sharp drop in air quality can only be explained by an incursion from outside," said an official.

A satellite image from NASA's Earth Observing System Data and Information System (EOSDIS) website on Saturday showed 'fire spots', which denote blazes on the ground, all across Punjab and parts of northern Haryana. Haze can be seen enveloping a large region of northern India, including NCR.

The NASA images are clear proof that measures taken by the Punjab government in discouraging the practice of stubble-burning have failed. Biomass burning is a major cause of air pollution in north India during this season.

According to another NASA website that maps fires, this season's stubble-burning in Punjab started around October 10 and reached its peak around October 30. Data for the last three days shows a sharp rise in the fires. During October and early part of November, farmers ready their fields for the sowing of the winter (rabi) crop.

Curiously, the images show the fires restricted to Punjab, except for parts of northern Haryana that border the state. Although stubble-burning is officially banned in Punjab, farmers continue to set fire to their crop residues twice a year because it's the cheapest way to get rid of the paddy stubbles.

The state government's subsidy for machines such as the happy-seeder and shredder, which collect the stubbles, haven't found many takers. Warnings and threats of prosecution also seem to have failed.

Pollution - from the farm fires and well as other regular sources - appears to have combined with weather conditions to cause the rapid fall in air quality over Delhi. Met officials said light easterly winds over the past couple of days have brought moisture into Delhi's air. Pollutants are known to stick to moisture droplets, causing smog.

"The relatively still conditions have allowed the smog to hang in the air. There has also been a steady fall in temperatures in the past few days that have lowered the boundary layer of the atmosphere," said B P Yadav, director, India Meteorological Department.

A lower atmosphere boundary does not allow pollutants to disperse into the higher levels of the atmosphere. In such conditions, once pollutants enter an area, they remain trapped close to the ground.

Yadav said the wind direction is likely to change from Monday or Tuesday. "That's when north-westerly winds will started blowing again," he said, raising the possibility of more pollutants coming in from Punjab.

There's little chance of rain in the coming week, Yadav added.