Why is there a saying in northern China that "spring rain is as expensive as oil"?

The area north of the Yellow River in China is controlled by cold air in winter and spring, and the rainfall is scarce and dry. Wheat in the field, rape and broad beans in some places almost stop growing in the cold winter. However, when spring returns to the earth and the Yan warbler sings, these wintering crops seem to wake up from their sleep and regain their vitality. For example, when spring comes, wheat begins to turn green, and its stems and leaves gradually turn from yellow to green. Adequate water is especially needed at this time. March and April are just the stage of preparing for bolting and flowering, and the demand for water is also urgent. Take broad beans, too. After spring comes, if there is not enough water, broad beans will die. In this way, in spring, water is particularly important for the growth and development of crops. At this time, if it can rain more, it is of course particularly valuable.

However, the influence of warm air in spring is still very weak, which does not affect the area north of the Yellow River. The main rain area still stays in Jiangnan. Rainfall in the area north of the Yellow River is still scarce, and there are not many opportunities for rain. There, the next spring rain is very gratifying, because the crops in the field need rain to moisten at this time, so there is a saying that "spring rain is as expensive as oil".