There are several grades of cold air.

According to the intensity, the cold air in China can be divided into three grades: weak cold air, moderate cold air, strong cold air, strong cold air and cold wave.

1, weak cold air: cold air that makes the daily minimum temperature in a place lower than 6℃ within 48 hours.

2. Moderate-intensity cold air: cold air that reduces the daily minimum temperature in a certain place by more than or equal to 6℃ but less than 8℃ within 48 hours.

3. Strong cold air: cold air that makes the daily minimum temperature in a place drop by more than or equal to 8℃ within 48 hours, but fails to make the daily minimum temperature in the place drop to 8℃ or below.

4. Strong cold air: cold air that reduces the daily minimum temperature in a certain place to 8℃ or below within 48 hours.

5. Cold wave: cold air that reduces the daily minimum temperature in a place by ≥8℃ within 24 hours, or ≥ 10℃ within 48 hours, or ≥ 12℃ within 72 hours, and reduces the daily minimum temperature in the place to below 4℃.

The birthplace of cold air

The cold weather in China is the result of frequent southward movement of Siberian cold air. Siberia is the "cold pole" in the northern hemisphere, which makes China the coldest place at the same latitude in the world. Siberian cold air appears as a cold air mass heading south on the weather map. But the cold air mass is not isolated, it is often a part of a larger weather system-cyclones on the weather map.

Cyclones are mainly composed of a pair of cold and warm air masses, with a cold air mass in the west and a warm air mass in the east. This extratropical cyclone rotates counterclockwise and moves from west to east at the same time. The dividing line between cold and warm air masses is called cold front.

After the cold front, the local area changed from warm air mass to cold air mass, and the wind direction changed from south wind to north wind. This is the truth of the proverb "the south wind blows to the end, and the north wind returns the gift". So there will be a sharp cooling after the cold front.