Atmospheric moisture that crystallizes directly on the ground and on exposed objects. The term also refers to the occurrence of subfreezing temperatures that affect plants and crops. Frost crystals, sometimes called hoarfrost in the aggregate, form when water vapour in the atmosphere passes into the ice-crystal phase without going through the intermediate liquid phase. Frost forms under conditions that would form dew if the temperature were above freezing. In agriculture, frost refers to the freezing of the water in plant cells, which causes the cells to burst and thereby destroys the plant.