Low Sky was the layer of the atmosphere which spanned from the ground to the altitude of the Sanctaphrax Rock. The categorisation by Low Sky, Middle Sky, High Sky and Open Sky was used to direct air traffic around Sanctaphrax but also by the sky-scholars.

Low sky phenomena like turbulent-fog were studied by the "less gifted", as Wilken Wordspool, Quint's teacher at the Fountain House, put it.

Later, in the Third Age of Flight, the expression "low sky" still persisted with the height being restricted by the highest Deepwoods trees as mentioned in The Immortals.