Ringvarju

Ringvarju is a meteorological optical phenomenon known in English as a halo. It appears as a circular ring of light surrounding the Sun or Moon, caused by the refraction, reflection, and dispersion of light by hexagonal ice crystals in high-altitude clouds such as cirrus or cirrostratus.

The most common form is the 22-degree halo, producing a ring with an angular radius of about 22 degrees. The inner edge of the halo is often reddish, while the outer edge tends to be bluish. Halos can occur alone or be accompanied by other features, such as sundogs (parhelia) positioned to the left and right of the Sun.

Other related halo types arise from similar optical processes and crystal orientations. These include circumhorizontal arcs, circumzenithal arcs, and various tangent arcs, which may appear as colored arcs or partial rings within or around the main halo. The term ringvarju in Estonian broadly covers these ring-like halos, though English-language sources more commonly use the umbrella term halo.

Formation and conditions: ringvarju forms when sunlight or moonlight passes through vertically oriented, plate-like ice crystals in thin, high-altitude clouds. The crystals act as prisms and mirrors, bending and dispersing light to create the circular band of illumination.

Occurrence and interpretation: halos are relatively common in regions where high-altitude ice clouds are present and can indicate approaching weather changes, though they do not provide precise forecasts. In Estonian usage, ringvarju reflects both the physical phenomenon and its visual essence, literally translating to “ring shadow.”