What does not affect visibility on the ground?
Mist is often defined as a visibility between 1000 and 5000m, and fog as a visibility below 1000m. The air is always containing some amount of water – either as invisible steam, as liquid water droplets or as solid water - like ice crystals, snow and hail. The humidity is depending on the type of air mass, as warm air can contain more water than a cold air mass. It will also depend on the track of the airmass. An airmass that have moved a longer distance over water will be more humid, than an airmass (with the same temperature) that have passed mainly over land. If the air is cooled down, the water vapor will condensate into water droplets resulting in mist or fog. This is the same process as in cloud formation, where air is rising and therefore cooled down until saturation is reached. The water vapor will then condensate into water droplets.
