For differentiating between dew and mist letus first understand fog.

Fog can be described as a cloud forming very close to or at ground level. It requires air with sufficient moisture to cool or to be cooled to a temperature to achieve condensation. Once condensation occurs, visibility decreases dramatically. It therefore becomes very hazardous.

Fog does not necessarily have to be uniform, or widespread. Some areas may have thick fog whilst others may miss out completely. Fog is more common in valleys and near creeks or streams. It also most commonly occurs overnight and during the morning. However, in colder climates where fog is more common, it may last all day.

Mist is similar to fog except it may appear more ragged and forms on rainy days with the air saturated due to the amount of moisture. Mist is more common in mountainous and forest regions.

Dew is moisture that condenses at the ground level. The layer of air within about a metre above the ground experiences rapid changes in temperature from day to night. Therefore the temperature may be cooler on the ground than the air above. Consequently, the air temperature close to the ground may cool to below the level needed for condensation. Moisture therefore is observed on grass and on the surface of some objects such as those with metal surfaces. If the air above also cools below the condensation level, fog will form.