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Perfect Lower Tangent Arc

Images from Italy by Dario Labella. Low level diamond dust crystals formed the halos.

An upper tangent arc on top of a 22-deg halo is a common sight. We see the lower tangent arc much less often and rarely as complete and perfect as this.

The bright spot at the bottom of the oval is a subsun.

Images ©Dario Labella, shown with permission
A singly oriented column crystal. It can 'rotate' i.e. take all rotational positions about the horizontal long axis.

Hexagonal column crystals form the lower and upper tangent arcs. As they slowly drift downwards relative to air currents, aerodynamic drag orients them so that their long axes are almost exactly horizontal.

Rays enter and leave side faces inclined at 60 degrees to each other.

The tangent arcs change shape dramatically with the sun's altitude. They join when the sun reaches 29 degrees to form the circumscribed halo.


The blue circle represents the common 22 degree circular halo. It is absent in Dario's images.