Why does pollution in Delhi increase during the months of Nov-Dec each year?
Delhi is the capital of India and one of India's largest city.  It has over 30 million residents and an estimated 100+ million vehicles.  Delhi is surrounded by the agricultural states of Punjab and Haryana.  During the period Sep-Nov, the states of Punjab and Haryana harvest rice they have grown during the summer months.  Post the harvest, in order to prepare the fields for sowing again they burn the left over rice husk and dried plants.  The month of November marks the onset of winter in the north Indian plains and during this time the north westerly winds stop.  The combination of high vehicle number, large scale burning of post harvest leftover and onset of winter with no winds results in the months of Nov-Dec being the most polluted in the year.