Rritur

Rritur is a fictional atmospheric phenomenon described in speculative fiction set on the planet Asteria. In various narratives, it manifests as a shifting halo or ribbons of pale blue and violet light that flicker near the horizon at twilight.

Origin and naming. The term rritur is attributed to the fictional Asterian language, where it is described as meaning "twilight veil." The concept is used consistently across several works in the setting to denote a distinctive sky display.

Physical description and proposed mechanism. In the stories, rritur is produced by diffraction and interference of sunlight by slender, highly oriented ice crystals within high-altitude cirrus-like clouds. Specific wind shear and humidity conditions align the crystals to create colored, shimmering patterns that move with the upper-air currents. The effect is non-destructive and visible only under particular atmospheric states.

Occurrence and appearance. Rritur is reported at latitudes with strong upper-atmosphere winds, often during planetary autumn and winter in the narratives. It can appear as arcs, ribbons, or halos and typically lasts from several seconds to a few minutes. Colors usually include pale blues and violets, occasionally with pinkish tints at the edge of the display.

Observational and cultural role. In-universe scientists treat rritur as a diagnostic of atmospheric dynamics and microphysical cloud processes. Culturally, it is described as an omen or symbol of transition in Asterian folklore, and it is sometimes depicted in art and poetry within the fiction. See also atmospheric optics, halo phenomena.