The observation of clouds plays an important part in weather forecast, the key of which lies in deciding whether it will be sunny or rainy in the near future. Since the rain falls from clouds, the probability of rain or even the kind of the impending rainfall are closely related to the condition of clouds.

The ancient Chinese people paid special attention to the observation of clouds. Sentences related with cloud observation, such as "The cloud will take on a similar color before it snows," can be found in Shi Jing (The Book of Songs or Poetry). In Guan Zi, another book and an important work of the Legalist school (a school of thought during the Spring and Autumn and Warring States periods (770-221BC)), mentioned, "If the cloud is flat, there will not be heavy rain. Without the accompaniment of clouds which provide water, the rain cannot last long and will end soon." Only by careful observation and sensible analysis can the conclusion be reached.

Among the folks in ancient times, there were lots of proverbs relating to clouds that served as a basis for weather broadcast, such as "thunderclouds often follow a windstorm" and "Cumulocirrus (a kind of large cloud) results in strong wind the next day."

Cloud atlases were indispensable in the identification of clouds. The earliest cloud atlases uncovered so far are Tianwen Qixiang Zazhan and Zhan Yunqi Shu, excavated in Mawangdui (in Changsha of Central China's Hunan Province) and Dunhuang (Northwest China's Gansu Province) respectively, and both of which were used as omens for military purposes.

Many cloud atlases can be found in literary works, such as Song Shi (The History of the Song Dynasty (960-1279)) and Tong Zhi Lue (also a historical book). Besides, various editions of Bai Yuan Jing are cloud atlases with captions, which remain in use even today.

Author: Jessie