On January 24, 2016, temperatures in Hong Kong fell to 3.3 degrees Celsius in some urban areas, and below freezing in the mountains. It was the coldest weather in Hong Kong for nearly 60 years. The extreme cold was also felt in other parts of East Asia and the US.
Many people wonder why we had such a cold winter when scientists are constantly warning us about global warming.
Global warming usually refers to a rise in average surface temperatures on land and sea over a long period, but it also means intense periods of cold weather, too.
We should note that a single cold snap in a small area does not disprove global warming. And we should bear in mind that cold winters are here to stay, regardless of whether the planet is warming up.
Moreover, we may experience more intense winters as the Earth warms up because a warmer planet will allow the air to hold more moisture than usual. During the past couple of years, extreme hot and cold spells have swept across Asia, Europe and North America.
The intensity and duration of these recent cold spells were extraordinary compared to those in the past few decades.
The cold period in south China during early 2008, and the severe snowy winter in European countries in 2009/2010 are also good examples of abnormal weather.
Another problem is that the substantial loss of ice over the Arctic Sea may impact the polar vortex. The westerly jet stream is the boundary between cold northern air and warmer southern air. This boundary is caused by the difference between temperatures in the northerly latitudes and the tropical ones. The Arctic is warming up faster than the global average, and the melted sea ice absorbs more of the heat from the sun than reflective ice does.
The narrowing north-south temperature gradient may weaken the jet stream and help the polar vortex escape and push the cold Arctic air flow southward. It was Arctic air flowing much farther south than usual that led to the extremely cold winter in Hong Kong this year.
After a sudden stratospheric warming, the pressure difference between the Icelandic Low and the Azores High is reduced. This pressure difference determines the prevailing wind direction for Eurasia and thus determines whether the Eurasian winter will be cold or warm.
It is suggested that Eurasia will experience decades-long periods of predominantly colder winters alternating with equally long periods of warmer winters.