Water, earth and air

[Jorge_Stolfi]

... or, as in this case, the element air.

I don't think the cloud-band represents the element "air".  IIUC, the Aristotelian view of the cosmos had five spheres: Earth, Water, Air, Fire, and Aether, from the center out (from bottom up).  The Aether was incorruptible and weightless and its sphere was the Heavens; it started just below the Moon and contained the stars and planets.  

This cosmology was distinct from the Biblical one, that had a solid "vault of the sky" above the stars, holding up a vast ocean of the "waters above".  Medieval Christian theologians turned themselves into knots in attempts to compatibilize the two views.  Anyway, AFAIK the cloud-band of medieval illustrations definitely represented the Biblical "vault of the sky" (above the stars) and  not the Aristotelian sphere of Air (below the sphere of Fire and the stars).

All the best, --stolfi