What is the meaning of "third class" in the 24 solar terms?

In ancient times, five days was a cycle and a solar term was fifteen days, so it was divided into three cycles. 365 days a year (average year) is 72 days, corresponding to 24 solar terms. Each climate corresponds to a phenological phenomenon, which is called climate response. Among them, the plant climate should be germination, flowering, fruiting, etc. Animals should start vibrating, singing, mating, migrating, etc. Abiotic climate should include initial freezing, thawing and thunder.

The time series changes of 72 climate events reflect the overall situation of climate change in a year.

The seventy-two changes originated very early and played a certain role in agricultural activities. Although some phenological descriptions are not so accurate and some are unscientific, they still have certain reference value for understanding the climate and its changes in ancient North China. Because the initial unit of phenology at that time was small and the actual climate and region were quite different, it was difficult to be widely used.