How does frost in early morning hours form?
Frost forms in much the same way as dew. Frost forms when a surface cools (through loss of infrared radiation) to a temperature which is colder than the dewpoint of the air next to the surface. Frost is frozen water that has condensed from some of the water vapor contained in the air. If a significant layer of air next to the ground were actually cooling to the dewpoint, then a visible shallow layer of fog would form. Instead, the air next to the surface (say grass) must be so thin that the only evidence that condensation has occurred is the frozen water on the grass.
