What is the phenomenon of extremely cold weather?

there is no extremely cold weather, but extremely cold weather, which means the weather below -4℃.

according to the "cold degree scale" made by meteorological specialty, the temperature ranges from below -4℃ to 9.9℃, and from low to high * * * is divided into eight grades. One of them is "extremely cold".

In recent years, in Europe and Asia, the snowstorm and severe cold have caused the lowest temperature in many areas in a hundred years. Moreover, heavy snow caused the interruption of air, rail and road transportation, and many towns lost contact with the outside world, power supply was interrupted, and natural gas supply was tight. In some areas of Inner Mongolia and Heilongjiang, the minimum temperature has also fallen below -4℃, which is related to extremely cold weather.