The "all-black snowfall forecast map" scared netizens! What is going on with this rare weather?

Recently, the "Henan weather forecast map is all black" has caused heated discussions on the Internet. Even the Central Meteorological Observatory's Weibo called it "outrageous", and many netizens said that they had never seen such a weather forecast map.

So, why is Zhengzhou's "snowfall forecast map" completely black? What does this mean? And what impact will it bring? Let's talk about it in detail today.

Image source: Weibo screenshot

30mm of snow doesn't sound like much.

Why is it so scary?

In meteorology, snowfall, rainfall, and rainfall + snow are all considered precipitation processes. In precipitation maps, different color scales are usually used to express different precipitation conditions. For example, the coloring of snowfall is usually gray, and when the snowfall is less than 5 mm, it is usually gray. The greater the snowfall, the darker the color. (For the sake of convenience, "snowfall" in this article refers to the precipitation after the snow melts.)

According to the forecast, the total snowfall in most areas of Henan during this rain and snow process will be more than 10 mm, and some areas in Henan may reach 20 mm or even more than 30 mm. It is no wonder that it is pitch black.

The previous precipitation forecast map for Henan Province from January 31 to February 5 showed that the diagonal area was sleet or rain turning to snow.

Many people may wonder: The snowfall is 30 mm, which is not deep, is it that scary?

Of course, we must know that this "30 mm" refers to the precipitation, that is, the water depth measured by the meteorological observation station after the fallen snow melts into water, and it is not the "snow depth" accumulated after the snow falls to the ground that we intuitively see.

When snowflakes fall to the ground, there are a lot of gaps inside, so the depth of the accumulated snow is much greater than the actual snowfall. Usually, the depth of snow is 8 to 10 times the amount of snowfall. In other words, 30 mm of snowfall may actually correspond to a snow depth of nearly 30 cm. This degree of snow depth will have a significant impact on transportation, daily life, etc.

But there is no need to be too anxious or worried, because the 30 mm precipitation level forecast this time does not refer to the total precipitation within 24 hours, but the total amount for six days from January 31 to February 5, which is about 5 mm per day on average. The corresponding snow depth is approximately equivalent to moderate to heavy snow.

Relationship between 24-hour precipitation and snowfall level at a single station.

For Henan, the heaviest snowfall occurred on the 31st and February 3-4. In addition, many parts of the country will also be affected by this large-scale rain and snow (especially freezing rain) weather. The Central Meteorological Observatory also analyzed that this is the most complicated meteorological condition for the Spring Festival travel since 2008. We need to pay close attention and take precautions, but there is no need to exaggerate the situation or panic.

Weather more dangerous than snow

Freezing Rain

In addition to snowfall in the north and rain in the south, there is another special and very harmful weather phenomenon that we need to pay attention to - freezing rain .

Under normal circumstances, the atmospheric temperature decreases with increasing altitude. However, when cold and warm air meet, the cold air with lower temperature and higher density tends to sink close to the ground, while the warm and moist air with higher temperature and lower density is lifted up. This "cold below and warm above" temperature stratification is called "temperature inversion" in meteorology.

At this time, when the initial snowflakes in the cloud fall, they first pass through the warm and humid air flow area to melt into water droplets, and then further fall through the not-too-thick freezing layer to form supercooled water droplets. At this time, if the ground (and the air close to the ground) temperature is below 0℃, these raindrops will quickly freeze into ice and cover the objects on the ground. This is the phenomenon of freezing rain.

From left to right: Schematic diagram of different temperature conditions for the formation of snow, ice pellets, freezing rain and rain (the red air layer is above 0℃, the blue air layer is opposite). Note the freezing rain process, that is, there is a melting layer in the air and the lower atmosphere and the ground are below 0℃, which goes through the process of snow → rain → rapid freezing.

The numerical forecast results of the temperature changes in the area around Wuhan with altitude (vertical axis) and time (horizontal axis). It can be seen that from February 2 to 4, the air temperature in this area at about 1000-3000 meters was higher than 0℃ (marked in red), but the lower atmosphere and the ground were below 0℃, which is very likely to form freezing rain.

If the freezing rain is strong, a large amount of frozen ice will make the road surface extremely slippery . If the accumulated ice weighs heavily on power and communication equipment such as wires and towers or railway transportation equipment such as overhead lines, it may also cause large-scale impacts on the power system, communication system and railway transportation system.

On February 1, a large number of trains on the Beijing-Shanghai High-Speed ​​Railway between Bengbu South and Suzhou East were slowed down or even stranded for a long time due to icing on the contact network, causing a significant impact.

In addition, ice frozen on the surface of crops and buildings will also have a significant negative impact on agriculture and daily life. The scope of this freezing rain event is also one of the largest in the past decade , and many areas have rarely experienced freezing rain before. Compared with heavy snowfall, the threat faced by freezing rain areas is greater and requires more attention and prevention.

Which areas will be hit by heavy snowfall and freezing rain?

What kind of impact does it cause?

For areas affected by heavy snowfall and freezing rain, local authorities need to pay close attention to the impact on road traffic, communications, power systems, agriculture, etc., and make timely plans and response measures. Individuals also need to pay attention to cold protection and reduce going out as much as possible (especially the elderly and children) . If you go out, whether driving, riding, or walking, you need to pay attention to anti-slip. If necessary, you can also prepare some emergency supplies.

In addition, it is the peak period of the Spring Festival travel rush. According to the forecast of the Central Meteorological Observatory, by February 5, friends passing through areas with heavy snowfall and freezing rain, especially those in important hubs such as Wuhan, Hefei, and Zhengzhou , need to pay close attention to weather, road traffic, train status, and other information, and prepare for response measures.

Snowfall and freezing rain will continue in many places in western Hubei and Hunan from February 6 to 8, so you need to be more careful if you plan to travel to these areas.

Based on the current forecast results of the meteorological department, the areas that may be severely affected by snowfall and freezing rain (blue areas) and railway traffic conditions are delineated. (Note: This map includes some planned railway lines)

I hope that all people affected by this large-scale rain and snow can take precautions and get through it safely; I also firmly believe that at the end of the long clouds, we will eventually be able to face the strong wind and greet the sunny day of the new year after the clouds and fog have cleared.

Planning and production

This article is a work of Science Popularization China-Starry Sky Project

Produced by: Science Popularization Department of China Association for Science and Technology

Producer｜China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

Author: Fengyun Mengyuan, PhD student in climatology

Review丨Kang Yanyan, Senior Engineer, Beijing Meteorological Bureau

Planning丨He Tong

Editors: He Tong, Lin Lin