Both the upper and lower tangent arc form when hexagonal rod-shaped ice crystals in cirrus clouds have their long axis oriented horizontally. Each crystal can have its long axis oriented in a different horizontal direction, and can rotate around the long axis.

Dec 16, 2024 · As the sun ascends above 29º, the upper and lower tangent arcs merge, forming a single halo that encircles the sun entirely. This halo is known as the "circumscribed halo," offering a captivating spectacle for skywatchers.

Dec 14, 2024 · Three phenomena in this tangent-arc category include the upper tangent arc, the lower tangent arc, and the circumscribed halo. We will examine the optics of upper tangent arcs in detail, and then briefly discuss the others.

Upper and lower tangent arcs and a faint 22º halo. HaloSim3 simulation. The region below the horizon is dark blue. Tangent arcs alter shape as the sun's altitude changes. Mouse slowly over the slider to see them. When the sun is on the horizon, both arcs are narrow 'Vs'.

The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low (less than 29–32°) it appears as an arc over the sun forming a sharp angle. As the sun rises, the curved wings of the arc lower towards the 22° halo while gradually becoming longer.

Dec 16, 2024 · This article explores the rare atmospheric optics phenomenon of Parry and Upper Tangent Arcs, discussing their formation, changes in shape with the sun's ascent, and the role of ice crystals in creating these captivating arcs.

When the angle of the Sun above the horizon is 30° or higher, the ends of the upper and lower tangent arcs will have curved around the Sun so much that they appear to join together to form a continuous oval shape. These joined upper and lower tangent arcs are then known as a ‘circumscribed halo’.

Upper arc. The shape of an upper tangent arc varies with the elevation of the Sun; while the Sun is low (less than 29–32°) it appears as an arc over the observed Sun forming a sharp angle. As the Sun is seen to rise above the Earth's horizon, the curved wings of the arc lower towards the 22° halo while gradually becoming longer.

Singly oriented column crystal with upper and lower Tangent Arc rays. The rays enter a prism side face and leave through another inclined 60º to the first. The best tangent arcs wing outwards from where they touch the 22º halo directly above and below the sun. Their colours are bright yet delicate. Red is sunward.

The most common of these are the upper and lower tangent arcs to the halo of 22°. They form separate arcs when the sun is low, but as the sun climbs they join to produce the circumscribed halo. These arcs are explained by refraction through the 60° prism sides of columnar crystals oriented with their long axes horizontal. See 46° lateral arcs.