Fog occurs in many and varied wondrous forms. A step towards appreciating its true glory is to be able to recognise some of the basic variations. A good working knowledge of Fog is also essential when trying to avoid unfortunate accidents while adventuring. Please join with me in making Fog Ontology a mandatory part of the six months introductory training that Guild offers all its members. For those who missed my last lecture on the subject, fog comes in three basic types:
Air Mist is formed by raising the dewpoint of a volume above the ambient temperature. In a breeze, the mist will disperse unless the Adept concentrates. Visibility is typically 30 feet.
Water Fog is a ground fog formed by increasing the amount of water in a volume until the air is saturated. The fog is not affected by air movements. Visibility is typically 5 feet.
Illusory Mist is formed from summoned water droplets on mana condensates which have been enchanted to be hygroscopic. The mana particles are not affected by air movements, leading to claims by other colleges that this fog is somehow less natural than other magical fogs. Visibility varies based on the skill of the Adept.
Illusory Fog is a larger form of Illusory Mist, with a much greater visibility which ironically makes it much more mist-like than Illusory Mist. It is the largest of the normal magical fogs, reaching up to ¾ of a mile across and over 50 feet high. As a ritual, it is almost impossible to dissipate the fog. Visibility varies based on the skill of the Adept.
Gaseous Form transmogrifies an entity into a thick slurry of vapour that roils and churns. It can usually be identified by (a) the corresponding absence of an entity; (b) its determination to contort itself or squeeze through small gaps; and (c) its very short duration. It is not a true fog.
Knockout Gas is sometimes seen as a translucent mist that occurs in cylinders 10 to 20 feet high and 30 to 120 feet across, and does not affect visibility. As the vapour suspended in the volume is a poison rather than water, it is not a true fog.
Noxious Vapour is a thick roiling white ground mist that is summoned from some charnel hell. It can be up to 10 feet high and many hundreds of feet across. Visibility is reduced proportionally to the power of the Adept. As the vapour suspended in the volume is a poison rather than water, it is not a true fog.
Mist Form is a natural alternate shape of Vampires and some other folk. Most Vampires travel at no more than their normal waking pace when in mist form, but can disperse themselves so widely that they can fill a room, and are very difficult to see, or concentrate themselves to little more than their usual size, forming the same thick slurry as a Gaseous Form. Mist Form uses the aqua vitae of the entity as the vapour, and so is not a true fog.
Ethereal Mist is found through out the Ethereal plane. Small amounts of it are also released into other planes when an Illusionist Maze spell is cast.
March Mist is found in the Middle Marches, the half-worlds of heath and swamp containing thin winding paths linking the inhabitable worlds. It is said that those who leave the paths between worlds simply vanish into the mists and are never seen again. The mists also apparently protect travellers from sights that can drive mortals insane. The mists may well be inter-dimensional, concealing the gaps in reality that constitute most of the Middle Marches. The slow portal to Bergelfen is a path through the Middle Marches.
Portal Mist is quite common when long-range or inter-dimensional portals are used. It is usually due to a leaking of inter-dimensional mist through the portal conduit. An example of portal mist occurs on the Elven Isles, where an old Elven portal to Terranova has been reactivated. Rings of standing stones, dolmen, or other framed portals often form with mist obscuring the destination. This is also known as portal mist.
Inter-dimensional Mist is the mist that lies at the interface between the planes, or perhaps is formed from the temperature or moisture differential between the planes. Several different examples can be found in the Inter-Plane Travel article.
Advection fog occurs when warm, moist air moves over a cold surface and the air cools to below its dew point. This typically occurs with a coastal evening breeze. It stops when the breeze dies, the temperature gradient flattens or the warm air loses sufficient water vapour.
Evaporation fog is produced when sufficient water vapour is added to the air by evaporation. The two common types are steam fog and frontal fog.
Frontal fog forms as warm raindrops evaporate in a cool air mass, often a cold front. The frontal fog dissipates when precipitation stops.
Steam fog forms when cold air moves over warm water. The fog is warmer than the surrounding air and may be quickly lifted, forming rotating swirls of fog or steam devils. This is the opposite of Advection fog.
Inversion fog is (usually) Radiation fog that lifts above the surface but does not completely dissipate because of a strong temperature inversion that exists above the fog layer. It is also known as Fog Stratus. Any wind dissipates Inversion fog.
Ice fog is any fog composed of tiny suspended ice particles. This occurs when dew point is below freezing, typically by rapid super-saturation of the air with water vapour from an external source.
Radiation fog is produced over land when radiation cools the air temperature. It is also known as Ground fog and Valley fog. It is the most common form of fog, and is dissipated by a slight breeze, or by air warming.
Orographic fog is formed as moist, stable air flows upward over a topographic barrier. Reversing or calming the wind or removing the topographic feature will end the fog – otherwise, they can last for many days. It is also known as Upslope fog.
Brocken Spectre A fearsome kind of Undead only found in fog or mist.
Condensates Small particles suspended in the air which provide surfaces upon which water vapour can condense to create cloud or fog droplets. Some particles, such as salt, are called hygroscopic as they will cause water to condense even when there is relatively little water.
Dew point The temperature to which air must be cooled for fog to occur. This varies according to the amount of moisture in the air.
Diffraction The bending of light around objects, such as cloud and fog droplets, producing fringes of light and dark or coloured bands, as well as areas which cannot be seen into.
Fogbow A phenomenon similar to a rainbow, but consisting of a single white arc of light which is brightest at the two points where it meets the sea or river from which it has extended. Sea or river fog is an excellent source of fogbows. The fog forms when air in contact with the cooler water is chilled. To see a fogbow, the fog layer must be sufficiently thin that the sun shines through.
Heiligenschein A faint white ring surrounding the shadow of an observer's head on a dew-covered lawn.
Mist Any fog that allows a visibility of over a mile is merely a mist.
Wet haze A haze of dust or salt particles in the air which is moist and obscures light noticeably, but is not composed of droplets of water.
The fog comes
On little cat feet
It sits looking
Over the harbour and city
On silent haunches
And then moves on.