Fog Or Mist?

For those of us who delight in terminological exactitude autumn is a time of year that presents us with a dilemma. When we open the curtains and look out and there is a veil over the landscape are we gazing upon a mist or a fog?

In simplistic terms, fog is a cloud that touches the ground, caused when water vapour, water in its gaseous form, condenses. During the process of condensation, molecules of water vapour combine to make tiny droplets of water that hang in the air. It is these that we see as fog because water vapour in its gaseous form is invisible. The volume of water in a fog cloud is around 0.5ml per cubic metre, the equivalent of 1.25 litres of water if you filled an Olympic sized swimming pool with fog and condensed it.

Fog and mist are formed in exactly the same way, the differentiator its impact on visibility. If you can only see less than 1km in front of you it is fog, whereas if you can see further than 1km it is mist. Confusingly, in weather forecasts for the general public the term fog is only used where visibility is less than 180m.

Haze, though, is a completely different kettle of fish, produced from a suspension of extremely small, dry particles in the air. These are invisible to the naked eye but en masse are sufficient to give the air an opalescent appearance. They also contribute to the red skies seen at dusk and dawn.

There are a number of different forms of fog. Radiation fog usually occurs in winter when the cooling of the land overnight by thermal radiation cools the air close to the surface. This reduces the ability of the air to hold moisture, allowing condensation and fog to occur, but it usually dissipates as the ground warms up again.

Valley fog forms when sold dense air settles in the lower parts of a valley and condenses, usually when warmer air passes above the valley. Advection fog when moist, warm air passes over a colder surface and is cooled while upslope fog is formed when winds blow air up a slope, the air cools as it rises, allowing its moisture to condense.

The most localised form of fog is evaporation fog caused by cold air passing over warmer water or moist land, causing freezing fog or, sometimes, frost. When some of the relatively warm water evaporates into low air layers, it warms the air, causing it to rise and mix with the cooler air that has passed over the surface. The warm, moist air cools as it mixes with the colder air, allowing condensation and fog to occur.