shǔ / jiǔ / kāi / shǐ / yǐ / hòu / běi / fāng / de / tiān / qì / yì / tiān / bǐ / yì / tiān / lěng数九开始以后,北方的天气一天比一天冷。After the nine periods begin, the weather in the north grows colder each day.