ground frost

noun

1. 1.The condition of air temperature dropping below the freezing point at or near ground level.
2. 2.Ice produced by the direct deposition of water vapor on grass, trees and other objects in freezing conditions.
3. 3.The frozen upper level of soil, caused by the air temperature dropping below the freezing point for a lengthy period of time; in temperate climates the soil thaws completely for the summer and in cold climates it may develop to permafrost.