New Delhi, Aug. 7 -- It took a rare confluence of five factors-strong easterly winds, rain the previous day, low dust and emissions, a high ventilation coefficient, and bright sunshine-for Delhi to wake up on Thursday to something it rarely does: a clean, clear, cerulean sky.

To be precise, the wind speed was 30-40kmph, 13 mm of rain was recorded at the Safdarjang station on Wednesday, the contribution of dust to the air pollution was nil, and the ventilation coefficient was 45,000 metres square per second.

A set of numbers in this ballpark-which on Thursday equalled a blue sky with a mix of interspersed high and low white clouds-coincide only a few times a year in the Capital.

The data above is from the India Meteorological Department a...