Beijing's first snowfall was not "up to standard"! How heavy a snowfall is considered the first snowfall?

Review expert: Ye Haiying, deputy director of the New Media Center of the Meteorological Publicity and Science Popularization Center of the China Meteorological Administration (China Meteorological News Agency)

On November 26, the first snow of this winter quietly fell in parts of Beijing. How is the snow scene in your area?

Image source: Beijing Daily, photo by Wang Jianzhong

It is understood that the earliest first snow day in Beijing in previous years was October 31, which occurred in 1987. According to the statistics of the first snow date from 1961 to 2022, the average first snow day is December 3.

Is this snow the first snow?

According to meteorological standards, the first snow day in Beijing refers to the date when a large-scale snowfall occurs for the first time in Beijing in the winter of that year. The first snowfall day that meets one of the following two conditions is defined as the first snow day in Beijing that year:

1. Snowfall was observed at more than 10 of the 20 artificial stations in the city.

2. Snowfall is observed at all five urban stations (Chaoyang, Haidian, Fengtai, Shijingshan, and Observatory), or snowfall is observed at three or more of the five urban stations, and the snowfall at at least one station is ≥ 0.1 mm.

According to statistics, from 23:00 on November 24 to 8:00 on November 25, the average precipitation in the city was 2.7 mm, and the average precipitation in the urban area was 2.9 mm, which was a light rain level. The maximum precipitation in the city occurred in Miyun Chazigou, reaching 9.2 mm, and the maximum precipitation in the urban area was 4.5 mm in Haidian Zizhuyuan; the maximum hourly precipitation intensity in the city was 2.6 mm/hour in Miyun Liuheyu (05:00 to 06:00 on the 25th).

Although the snow last night had drifted into the urban area compared to the snowfall in the mountainous areas in the past few days, the affected area was not large and the duration was short, so it still did not meet the standard of first snow.

Why does it snow suddenly?

Snowfall requires both sufficient water vapor and cold air.

If there is sufficient water vapor and cold air from the north, snow clouds will form at low altitudes. The clouds will block sunlight, affect the ground's ability to receive solar radiation, and cause the surface temperature to drop, thus forming snow.

Zhai Liang, chief forecaster of the Beijing Meteorological Observatory, explained that during the cold wave weather, Beijing was affected by a particularly strong cold vortex weather, and the entire Beijing was in the cold vortex cloud system.

After nightfall yesterday, additional cold air came down. Due to the low temperature in the high altitude, the water vapor reached saturation and caused condensation. It did not completely evaporate during the falling process, so this sudden snowfall was formed. "The low-level air is actually very dry. Considering that it will be evaporated during the falling process, we originally expected that there would be no snowfall in the plains." Zhai Liang said.

There are two sources of water vapor transport in the Beijing area. One is the water vapor transport from the South China Sea. If the warm and humid air currents from the south and the cold air from the north meet in Beijing, Beijing will see snowfall. The other is the return flow of water vapor from the Bohai Bay. The easterly air currents from the Bohai Sea and the northwesterly air currents meet in Beijing, which will also cause snowfall.

Although the first snow standards vary across the country, there are common difficulties in forecasting the first snow, because snowfall requires the simultaneous satisfaction of water vapor and cold air, which are indispensable. This is why the first snow is "something you can encounter but not seek".

The temperature has changed suddenly recently, so everyone should pay attention to keeping warm. Let us look forward to the arrival of the first snow together!