What are the conditions for the first frost to form?

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.

In addition, clouds hinder the radiation cooling of ground objects at night, and clouds in the sky are not conducive to the formation of frost. Therefore, frost often appears on clear nights, that is, when the ground radiation cools down strongly.

In addition, the wind also has an influence on the formation of frost. When there is a breeze, air flows slowly over the surface of supercooled objects, constantly supplying water vapor, which is beneficial to the formation of frost. But when the wind is strong, because the air flows quickly, the time to contact the surface of cold objects is too short. At the same time, when the wind is strong, the air in the upper and lower layers is easy to mix with each other, which is not conducive to cooling down and will also hinder the formation of frost. Generally speaking, when the wind speed reaches level 3 or above, it is not easy to frost.

Therefore, frost usually forms in sunny, breezy or calm nights in cold seasons.