Mist
Mist is a term used to refer to a phenomenon on the world of Scadrial. While it does indeed resemble ordinary mist it exhibits many unusual properties.
Composition
Mist is one manifestation of Preservation's power and thus is closely linked to Allomancy.
Pre-Ascension
The mists are the basis for the phenomenon known as the Deepness
Post-Ascension
The mists during the Lord Rulers reign appeared at night time.
Interactions
It will react to any Allomancer, swirling around him or her. The mists can be pierced by an Allomancer burning tin to reveal the stars above. In special circumstances Allomancers have been known to draw on the mists for power, using them as fuel for Allomancy even after their ingested metals have all been burned.
The mists will not enter any building and even a simple tent is enough to keep them out.
Mist will reject anyone pierced by a Hemalurgic spike.
After the Final Ascension, there are two types of mists, one representing Ruin (a darker mist) and other Preservation. Where the mists are, Harmony can more directly influence things.[1]