How is the frost formed, and what disasters will the first frost bring to crops?

In the early morning of cold season, grass leaves and clods are often covered with a layer of frost crystals. They shine in the rising sun and melt when the sun rises. People usually call this phenomenon "frosting". Looking through the calendar, every year from 10, there is always the solar term "first frost". We have seen snow and rain, but no one has seen frost. In fact, frost does not fall from the sky, but is formed in the air near the ground.

Frost is a kind of white ice crystal, which mostly forms at night. In a few cases, it can also begin to form before sunset when the sun is tilted. Usually, the frost will melt soon after sunrise. But when the weather is cold or in a cool place, the frost will last all day.

Frost itself is neither harmful nor harmful to plants. What people usually call "freezing injury" is actually the "freezing injury" when frost forms.

The formation of frost is not only related to the weather conditions at that time, but also related to the nature of attached objects. When the surface temperature of the object is very low, but the air temperature near the surface of the object is relatively high, there is a temperature difference between the air and the surface of the object. If the temperature difference between the surface of an object and the air is mainly caused by the radiation cooling of the surface of the object, when the warmer air contacts the surface of the object, the air will be cooled, and when the water vapor is supersaturated, the excess water vapor will be precipitated. If the temperature is lower than 0℃, the excess water vapor condenses into ice crystals on the surface of the object, which is frost. Therefore, frost is always formed under weather conditions that are conducive to radiation cooling on the surface of objects.