The atmosphere of a gyrealm organizes itself into a stable pattern of circulation cells driven by the habitat’s rotation. The pattern is the dominant control on interior weather, and because it is set by rotation rather than by season or terrain it is far more regular than any terrestrial equivalent — regular enough that residents navigate by it.

Cell structure#

In Coriopolis the atmosphere normally carries six circulation cells spaced around the circumference, each therefore about ten kilometers wide at the floor, surrounding a single central cell along the rotation axis that turns in the opposite sense and closes the circulation.

The count is not rigidly fixed. A cell will occasionally split, giving seven outer cells, or two will merge, giving five. Such configurations are unstable and tend to be temporary, and the atmosphere returns to six.

Transition zones#

The boundary between adjacent cells is the defining feature of the interior sky. Air rises along one edge of a cell and descends along the next, so a traverse from one cell to the next crosses an updraft, a transition, and a downdraft in succession.

Transition zones vary. Some are smooth. Others carry severe wind shear, strong enough to strip lift from an aircraft without warning and to tumble it. A glider caught in a shear-zone downdraft can lose several hundred meters in seconds, and at altitude the loss is not immediately perceptible — without instruments it is difficult to tell whether one is climbing or descending. Turbulence intensity is the practical cue that a boundary is near, and experienced pilots steer by it, turning toward smoother air and back again to hold the edge of a cell.

Prevailing surface wind#

Surface winds in a gyrealm are almost always westerly — that is, they blow from the west. The Coriolis effect within the rotating frame deflects surface air against the direction of rotation, and since west is the spinward direction, air moving against the spin is moving eastward and arrives from the west.

The regularity of this is a navigational resource. A resident who cannot see a landmark can orient by the wind alone. The standard mnemonic pairs it with the right-hand rule used for the north–south axis: curl the fingers of the right hand in the direction of rotation and the thumb points north, while the fingers point west — the direction the wind comes from, cooling the fingertips.

Where there is no breeze at the moment, vegetation records the prevailing direction. Trees are shaped by it, and moss tends to grow on the western side of trunks.