Fog exists if the atmospheric visibility near the Earth’s surface is reduced to 1 kilometer or less. Fog can be composed of water droplets, ice crystals or smoke particles. Fogs composed primarily of water droplets are classified according to the process that causes the air to cool to saturation. Common types of this type of fog include: radiation fog; upslope fog; advection fog; evaporation fog; ice fog; and frontal fog.

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