Those previously unseen numbers were hard to believe, but they did seem to match up well enough with the noxious soup we could see, smell and taste outside. We are all far more familiar with the specifics of air-quality measurement than we would like to be. Apart from the AQI readings above 700, we were quite struck to see the readings for the smallest and most dangerous sort of particulate matter, called PM 2.5, which can enter deep into the respiratory system. These are named for the size, in microns, of the particles. A reading at a controversial monitoring station run by the American embassy showed a PM 2.5 level of 886 micrograms per cubic metre; Beijing’s own municipal monitoring centre acknowledged readings in excess of 700 micrograms.

　　For perspective on that set of figures, consider that the guideline values set by the World Health Organisation regard any air with more than 25 micrograms of PM 2.5 per cubic metre as being of unacceptable quality.

　　Chinese authorities have complained about the American embassy's insistence on independently monitoring—and publicly reporting—Beijing’s air quality. And sometimes much is made of the vast differences between those readings and China’s own official ones, which are often less dire. Indeed, a key feature of one of those mobile-phone apps is the side-by-side comparison of those competing data-sets. (It is of course a bad sign that people here need more than one app to keep up with all this.)

　　But on a day like Saturday, the discrepancy between official readings and independent ones hardly seemed to matter; you didn't need a weatherman to know which way the ill wind blew. Or failed to blow, as the case may have been. One expert quoted by Chinese media attributed this spike in pollution to a series of windless days that allowed pollutants to accumulate.