September - rain You are here: Home Photo gallery Atmospheric optics Ice crystal halos Lowitz arcs

Lowitz arcs are formed when horizontally aligned plate ice crystals (responsible for the 22-deg. left and right parhelia), are spinning along their horizontal axes through the sky. (This effect is the same metastable state as that of a playing card, for example, which you hold at its long edge and then let it fall). As the spinning ice crystals have one more degree of freedom of rotation than those responsible for the parhelia, an arc is formed rather than a spot of light. Lowitz arcs extend from the 22-deg parhelia down to the 22-deg halo (the lower Lowitz arcs), and up between the circumscribed halo and the 22-deg. halo (the upper Lowitz arcs). The arcs are very rare, with only a few sightings per year by a team of halo observers. Lowitz arcs can also form in a 120-deg.parhelion, as these are caused by the same type of plate ice crystals. Then, the 120 deg parhelion has two crossing arcs passing through it, and the combination takes the shape of an hourglass. In the gallery, there is one photo which very faintly shows this effect. Note: It helps to sit back from the computer monitor and view the photos from some distance. Else, the arcs are very easily lost in the film grains and other details in the photo.

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