120°-Parhelia (EE18/19)

The 120° parhelia, formerly also known as paranthelia, lie at the height of the sun, 120° from the sun in azimuth. They mostly occur together with the parhelic circle and stand out as white bright spots from it. If the parhelic circle is also visible, the 120° parhelia are relatively easy to find. However, they do not always occur together with the parhelic circle. The parhelic circle can be created by both plates and column crystals. In contrast, the 120° parhelia occur only with plate crystals. With a parhelic circle created by column crystals, you will search in vain for the 120° parhelia. From the other halo types, it can be quite well determined by which crystals the parhelic circle is created. A bright upper tangent arc or circumscribed halo is a typical indicator of column crystals. The 120° parhelia can also occur without the parhelic circle. If bright sun dogs are visible, the 120° parhelia are also likely. They are diffuse and easily confused with bright clouds. A brightening should therefore be checked for the correct height and azimuth. Related halos are the Liljequist parhelia and the oblique arcs through the 120° parhelia. The moon can produce corresponding paraselenae at an azimuth of 120°.

The 120° parhelia are formed in suspended plate crystals that are horizontally oriented. Sunlight enters the upper base face, is reflected at two adjacent prism faces and exits through the lower base face. The horizontal deviation is always 120°, regardless of the angle of incidence. The ice crystals must be relatively thick for this, otherwise only a few light rays are reflected at the inner prism faces.