In early 2008, southern China suffered a severe snowstorm.
The main disaster areas of this ice and snow disaster are Guizhou, Hubei, Hunan and Jiangxi provinces.
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Under severe cold weather, the Observatory headquarters in Tsim Sha Tsui once recorded a temperature of 7.5 degrees, a new low since the beginning of winter, and Tai Mo Mountain fell to 0.2 degrees, and the high ground may freeze.
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Because the air in northern China is drier and precipitation is less than in the south, the northern provinces, where the weather is colder, are not affected.
As can be seen from Figure 1 (picture series below), freezing rain has fallen in many parts of South China.
Freezing rain not only makes the road surface icy and slippery, leading to traffic accidents or interruptions; it also freezes on the transmission towers, which greatly increases the load weight of the transmission towers.
How does freezing rain form?
During the formation of freezing rain, it begins as snowflakes falling from high-altitude clouds (see Figure 2).
During the falling process, the snowflakes meet the warm air layer with a temperature higher than the freezing point and completely melt into raindrops. When the raindrops continue to fall, they encounter a cold air layer with a temperature lower than the freezing point above the ground. At this time, the raindrops no longer condense, but is the formation of supercooled raindrops.
When supercooled raindrops come into contact with anything on the ground that is below zero degrees Celsius, they immediately freeze into ice.
This precipitation weather phenomenon is called freezing rain.
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How do snow disasters form?
Conditions for the formation of freezing rain
Freezing rain is usually related to the encounter between cold and warm air currents. The ground needs to have cold air flow below freezing point to enter below the warm air current (see Figure 2) to form a temperature inversion layer. In addition, the lack of particles in the atmosphere that can be used as condensation nuclei is necessary for the formation of freezing rain. conditions of.
The large-scale snowstorm in 2008 was related to the La Niña phenomenon, that is, due to the lower than normal sea surface temperature in the central and eastern tropical Pacific Ocean, the variability of atmospheric circulation, and the abnormally frequent cold air activities.
While the cold air from the north continues to move southward, moist and warm air from the South China Sea and the Indian Ocean is continuously transported to South China. The warm and cold air flows in South China and the area south of the Yangtze River, resulting in a large amount of precipitation for a long time, including freezing rain and Falling snow eventually led to this ice and snow disaster.
Why does Hong Kong have cloudy and cold weather at the same time?
The cloudy and cold weather affecting Hong Kong since mid-January 2008 is somewhat similar to the snow disaster in southern China.
Just look at the distribution of air temperature and wind direction over Hong Kong on January 29, 2008, and it will be clear at a glance.
Figure 3 shows that an inversion layer appears over Hong Kong below 800 meters, and the temperature rises instead of falling with the height, forming a configuration of warm and humid above and cold below.
Figure 4 shows the airflow below 800 meters above Hong Kong. The relatively humid and warm southerly wind blows above, but the cold northerly wind blows at a low altitude near the ground.
In fact, during the cloudy and cold weather, a temperature inversion layer continued to appear over Hong Kong.
▼On February 20, the weather in Hong Kong was cold and the temperature in many districts dropped to single digits▼
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▼Citizens went out for testing in the severe cold weather on February 21▼
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Why is there no freezing rain in Hong Kong?
Hong Kong's longest cold weather warning
There is no freezing rain in Hong Kong because the ground temperature is higher than freezing point (see Figure 5).
Taking the distribution of atmospheric temperature over Hong Kong at 8:00 am on January 29, 2008 as an example (see Figure 3), the atmospheric temperature below 4000 meters is much higher than freezing point.
Although there is no freezing rain in Hong Kong, the weather is still cold and rainy.
The Observatory issued a continuous cold weather warning within 26 days from January 24 to February 18, 2008, for 594.5 hours, the longest since the cold weather warning was created in 1999.
While the cold weather warning was in effect, the minimum temperature in Hong Kong was below 12 degrees Celsius most of the time (see Figure 5).
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Why does the temperature fluctuate?
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Original: The 2008 Snow Disaster in South China and the Cold Weather in Hong Kong