Cold air can cause the temperature to drop at the sites. It is divided into five grades: weak cold air, moderate cold air, relatively strong cold air, very strong cold air, cold wave.
Weak cold air: it can make the place affected have a less than 6 °C drop in the daily minimum temperature within 48 hours. Moderate cold air: a 6-8 °C drop within 48 hours.
Relatively strong cold air: a more than 8 °C drop, but the daily minimum temperature above 8 °C within 48 hours.
Very strong cold air: a more than 8 °C drop, and the daily minimum temperature below 8 °C within 48 hours.
Cold wave: a more than 8 °C drop within 24 hours, or a more than 10 °C drop within 48 hours, or a more than 12 °C drop within 72 hours, and the daily minimum temperature below 4 °C.
Strong cold wave: a more than 10 °C drop within 24 hours, or a more than 12 °C within 48 hours, or a more than 14 °C within 72 hours, and the daily minimum temperature down to below 2 °C.
Very strong cold wave: a more than 12 °C drop within 24 hours, or a more than 14 °C within 48 hours, or a more than 16 °C within 72 hours, and the daily minimum temperature below 0 °C.
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