Why is there dew on the leaves?

Because above 0 degrees, the temperature at which air is saturated with water vapor due to cooling is called "dew point temperature". In warm seasons, when the ground objects are cooled by intense radiation at night, the temperature of the air in contact with the surface of the objects drops to the "dew point", and excess water vapor is precipitated. Because the temperature is above 0 degrees at this time, these excess water vapor condenses into water droplets and adheres to ground objects, which is dew.

Knowledge expansion:

Dew, like frost, mostly appears on clear, windless or breezy nights. At the same time, objects that are prone to condensation are often objects with large surface area, rough surface and poor thermal conductivity. Sometimes dew will form in the middle of the night, and the temperature will continue to drop in the second half of the night, making the dew on the object freeze. It's called frozen dew.

Some people classify it as frost, but its formation process is different from frost. Dew usually forms at night. After sunrise, the temperature rises and evaporation disappears. Dew often appears in crop growing season, which is beneficial to agricultural production.

In summer in northern China, water vapor evaporates quickly. When there is little rain and drought, the leaves of crops sometimes curl and are dried by the sun during the day, but when there is dew at night, the leaves will return to their original state. People often call it "rain dew".