The solar system in which John Harper’s Lady Blackbird takes place (the Wild Blue) is described in these terms:
The worlds of the Wild Blue float in a sky of breathable gases circling a small, cold star. … The heavier gases [known as the Lower Depths] form a dense layer of fog below the “sky” of the Wild Blue.
Trying to imaging the exact meaning of these sentences I came up with a few possibilities:
A. The breathable gases form a sphere encircled by a “shell” of heavier gases (darker in my sketches). You go “down” whenever you move away from the central star. You go “up” when you move towards the star.
B. The breathable gases surround the orbital plane of the worlds of the Wild Blue. If you move perpendicularly away from the orbital plane, you go “down” towards the heavier gases (so two opposite directions are both “down”). You go “up” when you move toward the orbital plane of the Wild Blue worlds.
C. The breathable gases surround the orbital plane of the worlds and extend in one of the two directions perpendicular to it. If you move in the opposite direction, you go “down”. Thinking in terms of the directions on the Earth, one could say that moving North you go up through the breathable gases, while moving South you go down. The Wild Blue is made of two hemispheres: an upper (Northern) one made of breathable gases and a lower (Southern) one made of heavier gases. You go “up” when you move North from the orbital plane.
I think that the idea I prefer is C, which is also the weirdest in physical terms. It possibly has some implications on the structure of the worlds too, since each world will have a Northern hemisphere facing the breathable gases and a Southern hemisphere facing the heavier gases. I guess that the quality of the air and the color of the sky could be different in different places of a planet.