© 2018 Royal Meteorological Society The thick smog that blanketed India's capital, New Delhi, in early November 2017 saw air quality index values peak above 1000 – a figure in excess of three times the threshold value for ‘hazardous’ conditions. A public health emergency was declared. Delhi's smog was the result of an existing ambient urban air-pollution problem, significantly worsened by smoke blowing in from numerous agricultural fires burning across neighbouring Punjab and Haryana states. Post-summer monsoon regional air-flow patterns, decreasing autumn temperatures, high-pressure stability, temperature inversion and light local winds helped to produce climatic conditions that were conducive to smog build-up and subsequently prevented it from readily dispersing. Well-intentioned measures introduced by the authorities saw only partial improvement in city air quality after three weeks. To reduce the severity of future smog hazards, a region-wide agreement to restrict stubble burning during late autumn across northwest India will be needed.