India, Nov. 13 -- For most of this week, Delhi breathed from a shrinking room. On Tuesday afternoon the city's smoke, dust and exhaust had 1,411 metres of air above the ground to spread through; by Thursday that ceiling had dropped to 396 metres, lower than on a usual afternoon in early January, and PM2.5 across the city more than doubled in a day.

That ceiling is invisible from the ground, but a weather balloon can find it. The India Meteorological Department (IMD) lets one go from its Safdarjung observatory at about 4.30am and 4.30pm every day, and on its way up it radios back what the air is doing at every height: how warm it is, how far the day's pollution has been stirred upwards, and how hard and from which direction the wind is bl...