Circumscribed halo/tangent arc

The circumscribed halo and tangent arcs actually have the same cause. One transforms into the other according to the height of the sun in the sky. The circumscribed halo is a halo in which, instead of being circular like 22º, it has an elliptical, oval shape. It is located outside the halo 22 °, except at its upper and lower points where the two touch (if halo 22 ° is present).

Tangent arcs are characterized as isolated parts that "detach" from the halo 22º, tangent to them. What tangents the halo at its upper point is called the upper tangent arc, and what tangents it at its lower point, the lower tangent arc.

With the sun on the horizon, the upper arc is formed, very curved (the lower one may be present if viewed from a high point, above or within the region of ice crystals, such as an airplane). As the sun rises, the upper tangent arc decreases its curvature and, with the sun at 22 degrees, the lower tangent arc begins to appear on the horizon.

As the sun is higher at approximately 30 °, the upper and lower tangent arcs touch outside the sides of the 22 ° halo. At this moment the tangent arcs lose their identity, and the circumscribed halo is formed. The circumscribed halo gets closer and closer to the 22º halo as the sun rises higher, and the two join with the sun near the zenith (the middle of the sky), forming a single halo.

The ice crystals responsible for the formation of the tangent arcs and the circumscribed halo are the column-shaped hexagonal section, which keep their longest horizontal axis (lying columns). The crystals can rotate around the axis, but they will always be lying down.

The formation of the circumscribed halo and tangent arcs is complex because incident rays can penetrate a slanted crystal at any angle. It is beyond the scope of this page to provide a detailed explanation of its formation. The idea was to present the basis of its formation and what form of crystal is responsible for these halos.