The snow yesterday was the earliest since 1987 in Beijing. According to statistics of the last fifty years, the first snow of the year would usually fall around November 29th, but this year it fell a whole month earlier.
Why did the snow come so early this year? The conditions for water vapor were good. The wind started blowing from the east yesterday and brought vapor from the Bohai Sea hundreds of kilometers east of Beijing all the way here. A rough estimate shows that if the average snowfall is five millimeters across the city, which qualifies as heavy snow, and with an area of 16000 square kilometers, there will be about eighty thousand tons of precipitation, which is quite considerable. The second reason is strong cold air. If the condition for water vapor was only sufficient, then it would have rained instead of snowed. The continuous flow of cold air from the south since the day before yesterday has cooled the air in the Beijing region enough to condense the water vapor into snowflakes.
Such conditions alone could not have produced such a heavy snowfall in Beijing. So why did it snow so heavily? Artificial snowfall. The Beijing Office for Artificial Weather Modification conducted artificial snow enhancement operations on this snowfall yesterday, so the snow was both heavier and lasted longer.