It was an auspicious start to what China wants to be a very good year. Beijing awoke New Year's Day to a cloak of white - thanks in part to the hard work of its weather engineers.
As the rest of Beijing rushed home on New Year's Eve, Cao Xue-cheng and his colleagues at the Beijing Weather Modification office were busy shooting liquid nitrogen and silver iodide into clouds drifting over the city.They were trying to coax every last drop of moisture out of the air and ensure that Beijing would have a thicker blanket of snow.
It's all part of a science China increasingly is relying upon to provide rain and snow to its thirsty northern plains and to prevent damage caused by hail and fog.
Beijing newspapers praised the Dec. 31-Jan. 1 snowfall as an "auspicious snow" to begin the new year. Since 1997 is the year the British colony of Hong Kong reverts to Chinese control, good omens are especially welcome.
Because China's leaders tend to take a personal interest in the weather, Beijing's Weather Modification Office has more resources and better equipment than many provincial offices.
The office opened in 1973. Its first task was to minimize the damage from hailstorms. It also has begun, on an experimental basis, using liquid nitrogen to disperse the woolly fogs that tend to settle on Beijing's Capital Airport each winter.
But since 1990, the meteorologists have focused their efforts on increasing precipitation in parched metropolitan Beijing.
Weather radar measures the density and height of the clouds. If the temperature is right, the meteorologists set to work.
In the summer, when thundershowers occur almost daily and the water is most likely to run off into local reservoirs, the office leases or borrows airplanes. It also has 37-mm anti-aircraft guns positioned around the city to shoot silver iodide into clouds expected to shower damaging hail on the city and nearby fields.
In the dry winter months, keeping airplanes on standby for an occasional snowstorm is too costly, and the anti-aircraft guns are in storage. So the engineers rely on hydrogen balloons rigged with firecrackers on long fuses to carry the pellets of liquid nitrogen and silver iodide into the clouds and disperse them.
The chemicals' molecular structure makes them likely to bond with the moisture in the clouds, increasing the probability and amount of precipitation. In the case of hail, ice particles tend to melt or fall to the ground before becoming big enough to cause damage.
Chinese scientists tend to prefer liquid nitrogen, Cao says, because it works more consistently at higher temperatures, is cheaper and causes no pollution. Silver iodide used in cloud seeding has been found to cause some pollution, although he said it is within safe limits.
Similar projects are underway throughout China, particularly in the dry north.
Beijing is naturally arid. Rainfall measures only 24 inches a year. Since the 1980s, population growth and increased farm and industrial use have drained ground water supplies.
Drought in recent years has often left local reservoirs dangerously dry. Projects are planned to divert water northward to Beijing from more humid areas to the south, but they are far from completion.
"Large-scale construction projects can take decades," Cao says. "When it comes to cloud resources, the cost is low and the supply is unlimited."