On a clear night, with no cloud cover, the ground temperature drops and cools the layer of air in contact with the ground through conduction. When the relative humidity of the air reaches 100% (which will occur at the Dew Point temperature), the moisture in the air may begin to condense and small droplets of water appear on the ground.
If dew has formed, and the temperature of the air immediately over the ground cools further below freezing (0 °C, 32 °F), the dew may not keep its liquid state but rather freeze to form frozen dew. During the freezing process, the water releases latent energy. In certain areas of the world gardeners may even spray water over their plants when temperatures are expected to fall below freezing in order for the plants to absorb the latent heat released during the freezing process, helping the plant keep its internal temperature at sustainable levels.
NOTE: Frost (Hoar Frost) is not frozen dew. Hoar frost develops not through condensation but rather through sublimation (deposition) of water vapour directly into ice on ground.