Airs are what Fallen London calls most of its randomizers within the game that affect what choices you are able to make.
Many choices that that you are able to make are limited by what the current Airs is. Most commonly this is the The Airs of London though many other areas and sub areas have their own Airs that dictates what choices can be made in those areas. Airs are preserved upon leaving the sub area. Most Airs are on a scale of 0-100, though some exceed this range.
Airs can be cycled by taking an action that has a particular Airs as a requirement, though there are some actions not dependent on them that also cycle a particular airs. Check on the individual quality for more details on those actions.
Action results may sometimes have variant text based on the value of Airs after taking the action.