Plumb Cliff Sky

Latin plumbum, lead — referring to the lead-weighted plumb-line of pre-modern carpentry. As a color modifier, plumb implies a clear-and-vertical-true quality where the hue carries the visual register of gravity-aligned-and-perfectly-vertical surface. Sits at the crisp-and-balanced end of the grid, parallel to level and squared in usage.

Old English clif, steep face of rock. As a color modifier, cliff implies a vertical-and-weathered quality, the visual register of Cornish-coast-and-Yorkshire sea-cliff sandstone-and-granite headland-face surfaces standing against open sky and salt-air in late-afternoon raking light. Sits at the modifier-and-place end of the grid, parallel to bluff and crag in usage.

The blue of a clear sky at noon — produced by Rayleigh scattering, the preferential dispersion of shorter wavelengths through atmospheric molecules. Air itself is colorless; the color we see is sunlight scattered toward our eyes by every cubic kilometer above. The reference shade is mid-latitude noon under a high pressure system: a clean, slightly green-shifted blue with the luminous depth of light scattered across an entire hemisphere of air.