Welkinstead was terraformed by accident. It turns out that oxygen is a byproduct of the gas refining process, meaning the more the platforms polluted, the more breathable the air became. Eventually a layer of air at normal temperature and pressure formed and is continuously maintained by the same cities the Welkinsteaders live in.

However, a layer of poisonous gas is below the level at which the cities can be found, and upwellings of this gas into the breathable layer are caused by cyclonic storms. When cities traverse these upwellings everyone has to go inside and the city has to seal up.