The weather on October 1 was picture perfect; sunny, blue skies, a crisp breeze and not a whiff of the photochemical smog that often cloaks the city. It was just right for the five-starred, red flag to snap commandingly in the breeze on the 55th anniversary of the founding of the People's Republic of China.
September 30th, however, was awful; grey, polluted, with low cloud cover and, towards the afternoon, finally, rain; cold, lashing rain. That is when the rumours began. 'I bet they're messing with the weather again, just like they did five years ago when China turned 50,' some people grumbled as they fought for taxis in the rush hour.
If weather control seems improbably Orwellian, think again - cloud-seeding, to produce rainfall, is common in the mainland. It is also practised in the US, where the science was developed in 1946. Australia and Russia do it, too. Last year, Beijing invested 413 million yuan in making it rain.
Call it what you will - cloud-seeding, rainmaking, the more sinister-sounding 'weather modification' or the plain coy 'precipitation enhancement', rainmaking has become the norm across northern China.
According to the China Meteorological Administration, cloud-seeding produced 10 billion cubic metres of rain in the first six months of the year. 'I believe that nearly every time it has rained in Beijing or northern China this spring, cloud-seeding technologies have been used,' Hu Zhijian, of the administration's Institute of Artificial Rain, told the China Daily.
The technology is simple. Meteorologists identify moisture-bearing clouds and scatter a small amount of chemical - usually silver iodide - into the dense mass. More than 3,800 rocket launchers, 7,000 anti-aircraft guns and 'many airplanes' were used in the first half of this year to achieve this, meteorologists say. The silver iodide crystals act as nuclei, around which moisture collects in the form of ice. This becomes heavy and falls, turning to water once it passes below a certain temperature threshold. Simple, in theory.
In reality, despite the government's large outlay, scientists in China and elsewhere are not entirely sure whether it really works, and the fact that efforts continue reflects the growing pressure on water resources worldwide.
But because rain is so variable from day to day and from place to place, it is very hard to tell if seeding was the cause of rainfall, or something else.
As for the mystery of the great weather on National Day - it probably was just a coincidence. After all, even cloud-seeders, those modern-day rainmakers, would have had problems creating the wind that blew away the smog.