For several weeks each winter the Indo-Gangetic plain, including all of Punjab, is under dense fog, with visibility falling below fifty metres for days at a time.
Flights are cancelled, trains run many hours late, and the highways become dangerous. Punjab's road fatality figures spike.
The fog is worse than it was, and the reasons given are irrigation, which puts moisture into the air, and particulate pollution, which gives the moisture something to condense on. The stubble burning of November leaves particulates that contribute into December.
Wheat likes it. A long cool moist winter is good for the crop, and a warm dry one is not, which is why the fog is a hazard and its absence is a bigger one.
Rural life adjusts: work starts later, weddings are scheduled around it, and the elderly stay in.
It is the most predictable annual disruption in northern India and there is very little serious infrastructure response to it.
The dense winter fog forms over the whole plain from late December. It closes airports, stops trains and kills people on the highways.
It is thickened by particulates. Fog needs nuclei to condense on and the plain now supplies them in quantity, which is why the fog season has lengthened over the decades it has been measured.
It also protects the wheat. The cool damp weeks are what the crop needs at grain filling, and a warm dry January is worse for the harvest than a foggy one.