Dew

When moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in the air above the surface), such as stones, grass blades and plant leaves, it is known as dew. The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights. For the formation of dew, it is necessary that the dew point is above the freezing point.

Frost

Frost forms on cold surfaces, when condensation takes place below freezing point (0°C), i.e., the dew point is at or below the freezing point. The excess moisture is deposited in the form of minute ice crystals, instead of water droplets. The ideal conditions for the formation of white frost are the same as those for the formation of dew, except that the air temperature must be at or below the freezing point.