Soul-searching question: How can we have a satisfying snowfall?

Recently, our friends in the south have not seen much snow or rain. Is it really that difficult to have a good snowfall?

↑In the past 10 days, precipitation in most areas of the south has been more than 80% lower than the same period last year.

Especially in the past few days, the prevailing northerly wind blew away all the moisture from north to south, causing most of the country to be dry. Even in coastal areas such as Fujian and Guangdong, the relative humidity was less than 20%.

In particular, due to the continuous lack of rainfall in Guangdong, the water levels of major rivers are significantly lower than in previous years, and many parts of the province are facing drought challenges. According to media reports, the Guangdong Provincial Government has recently issued a mobilization to fight severe and long droughts. The Shenzhen Water Authority also held a media briefing and stated that Shenzhen is facing the most severe drought since the establishment of the city.

Waiting for the snow that may fall at any time has become the most anticipated thing for our friends in the south recently.

The question is, is it really that difficult to have a good snowfall?

When water vapor in the atmosphere condenses or water droplets solidify directly, they become snowflakes. When they continue to aggregate and grow larger, until the air can no longer support them, they will fall from the sky and form snow.

Therefore, the key to snowfall is water vapor, temperature and dynamic conditions. It can be said that in most cases, snowfall is the result of careful "cooking" of cold air and warm and humid air currents. The former creates the right cooking conditions - temperature, and the latter provides the basic ingredients - water vapor. Combined with dynamic conditions, boom, boom, boom, snowflakes come.

Therefore, the "pain points" of snow in the south and the north are very clear. The main factor that holds snow back in the south is the temperature, while in the north it is water vapor.

Generally speaking, snowfall starts in October in the north, while in the south it starts to increase around January or February. This is because the temperature in the south is basically at its lowest point in winter, and combined with the moisture from the Bay of Bengal brought by the southern branch trough, there is a greater chance of snowfall.

The above describes the basic situation of snowfall. The situation varies across the country, and there are also several special "big snowfall areas".

●For example, the northern Xinjiang region.

This is the outpost of the western cold air that affects our country. Almost every time the cold air comes, rain and snow are likely to occur in the northern Xinjiang region. Take Urumqi as an example. The annual average number of snowy days in the past 30 years from 1991 to 2020 was as high as 64.2 days, making it the place with the most snowy days among provincial capital cities in the country.

Although located in the northwest inland area far from the ocean, the moisture conditions in northern Xinjiang are much better than other dry northern areas. Because the moisture from the Atlantic Ocean and the Caspian Sea comes with the westerly wind, and the moisture from the southwest from the Arabian Sea gathers in the Aral Sea area and is then transported to northern Xinjiang. When cold air arrives, the terrain lifting of the Tianshan Mountains and the Altai Mountains assists, making snowfall more likely.

↑On November 1, 2021, Bayingbuluke in Xinjiang welcomed snowfall. The accumulated snow contrasted with the mountains, and the sky and earth were vast.

●Another example is the eastern part of Inner Mongolia and the Northeast region.

In November, three cold waves caused rare heavy snowfalls in eastern Inner Mongolia and northeastern China. Tongliao, Inner Mongolia, broke the snow depth record set in 1951 with 61 centimeters; Fuxin, Liaoning and Shuangliao, Jilin, even doubled the snow depth record; Northeastern Heilongjiang experienced heavy to extremely heavy snow, with snow depth generally reaching 25 to 49 centimeters.

There are special factors for these heavy snowfalls. During these three cold wave processes, there were high-altitude cold vortices or high-altitude troughs over the Northeast, which cooperated with ground cyclones to import water vapor from the Yellow Sea, Bohai Sea and even the Sea of ​​Japan into the Northeast, providing abundant water vapor for snowfall. In particular, the Tongliao area is located in the transition area from the Mongolian Plateau to the Liaohe Plain. During the eastward movement of the cyclone, the southeastern water vapor was also affected by the terrain uplift, resulting in extremely heavy snowfall with snow accumulation exceeding 60 cm in the Tongliao area.

For our friends in the south who are "thirsty for snow", such fierce snow is really something they both envy and fear.

●For example, places like Weihai and Yantai in Shandong are known as “snow nests”.

↑Starting from December 29, 2020, Wendeng District, Weihai, Shandong Province experienced continuous snowfall. Thick snow covered the entire city. The roads were severely snowed, visibility was low, and pedestrian travel was hindered.

There is a unique form of snowfall here - cold stream snowfall.

↑The picture shows the cloud map at 14:00 on March 30, 2019, with cold current and snowfall in Yantai and other places.

After November, the strength of cold air gradually increases, and the air temperature and sea temperature drop rapidly, especially the air temperature drops faster, the temperature difference between sea and air increases, the condensation height above the sea surface decreases, and snowfall is more likely to form; when the cold air passes through the relatively warm and humid Bohai Sea and the Yellow Sea, affecting Weihai, Yantai and other places located on the windward slope of the north side of the Shandong hills, the cold air will be forced to rise with water vapor, thus making these places known as "snow nests".

Where there are “big households” that suffer from snow, there are “poor households” that suffer from snow.

From a climatic perspective, the temperature conditions are there, and it is more difficult to snow the further south you go. For example, in southern China south of the Nanling Mountains, snow is basically just a dream.

But there are exceptions. For example, the "super-cold wave" in 2016 broke the record and pushed the snow line south to the Pearl River Delta region. Snowflakes and ice particles (snow seeds) moved south with the cold wave, reaching the southernmost line of Shenzhen-Hong Kong-Zhongshan-Taishan in the Pearl River Delta region, and "captured" Qinzhou, Fangchenggang and Dongxing in Guangxi.

This broke the record for the southernmost snowfall in my country since 1951. According to historical documents, the last time such large-scale snowfall occurred in South China was during the Republic of China (1929) and the late Qing Dynasty (1893 during the Guangxu period).

This snow is a miracle.

In most cases, it is not the stronger the cold air, the better it is for snow to fall. If the cold air is too strong, the warm and humid air will be defeated before it can resist. The dry and cold north wind will prevail from low to high altitudes, and the water vapor will be blown away completely, so there will be no snowflakes at all.

For example, the southern part of China suffered a large-scale rain, snow and freezing disaster in early 2008. This was due to the continuous force of multiple rounds of cold air, none of which was particularly strong. This led to a continuous tug-of-war and confrontation between cold and warm air currents, which in turn led to rain, snow and freezing disasters that lasted for a long time and covered a large area.

But for South China, only strong cold air can make snowflakes fall in Lingnan. The "super-cold wave" in 2016 was extremely strong. After the temperature dropped, the lowest temperature in the middle and lower reaches of the Yangtze River approached minus 10 degrees Celsius. The 0 degrees line crossed the Nanling Mountains and penetrated into South China. The center of the cold high pressure of 1042.5 hPa directly advanced to northern Guangdong.

What is even more coincidental is that under the background of the large circulation at that time, the water vapor conditions in southern China were actually not good. But when it affected Guangdong on the 23rd and 24th, although the low-level air was indeed very dry, southwesterly airflow appeared at an altitude of 700 hPa in the middle and high levels, providing the water vapor and dynamic conditions required for this snowfall.

Therefore, on the day of January 23, 2016, snow or sleet first appeared in northern Guangdong. On the morning of the 24th, hail and sleet appeared in central Guangdong including Guangzhou. During the day of the 24th, hail, sleet or light snow was observed in Shenzhen, Zhuhai, Hong Kong and Macao. Many stations in central and southern Guangdong broke the observation records since meteorological records were kept.

↑On January 2, 2016, snow fell in many places in South China, and the snowman attracted many people to watch and take pictures.

It is a consensus that it is difficult to snow in South China, but Beijing, located in the north and not far from the Bohai Sea, is not a big snow country.

Beijing is surrounded by mountains on both sides, and only the east and south sides of the plain area are not blocked by mountains. Therefore, the only way to have a good snowfall is to rely on the easterly airflow of the low-level easterly return flow to bring water vapor from the Bohai Sea to Beijing!

↑On the morning of December 9, 2021, it rained in Beijing city and snowed in Huairou.

To summarize with the words of Hu Xiao, chief meteorological analyst of China Weather: The probability of snowing in Guangdong is low, and the difficulty is high, so it is something you can only encounter by chance; Beijing has many chances of snowing, but the variations are great, so it is better to encounter it than to foresee it; the probability of snowing in northern Xinjiang and the Northeast is high, the opportunities are many, and the amount of snow is also heavy, so if you encounter it, you have to see it.

For those who are still waiting for snow, why not keep this expectation and wait together for a day when "the mountains are covered with snow when you open the door in the morning"