Frost

Frost forms when condensation happens on a surface that is colder than the freezing point of water. The key difference from dew is the temperature at which the moisture in the air changes state. When the dew point — the temperature at which air becomes saturated — is at or below 0°C, the excess moisture does not condense into liquid water droplets. Instead, it is deposited directly as minute ice crystals. That deposit of ice crystals on a cold surface is what we call frost.

The physical condition that separates frost from dew is simply the temperature of the air at the surface. For dew, the dew point lies above freezing, so water vapour condenses into liquid. For frost, the dew point is at or below the freezing point, so the water vapour skips the liquid stage and turns straight into ice. This is why frost appears as a white, crystalline coating rather than as droplets of water.

The ideal conditions for white frost to form are exactly the same as those for dew formation, with one crucial difference. Both require clear skies, calm air, and high humidity near the ground, so that the surface cools rapidly by radiation on a clear night. The only extra requirement for frost is that the air temperature must be at or below the freezing point (0°C). When all these conditions come together, the moisture in the air deposits as ice crystals on exposed surfaces like grass, leaves, and rooftops. …