Low Sun Tangent Arc

A series of images at roughly five minute intervals shows how rapidly haloes can change appearance, particularly ones which are sensitive to the shape and orientation of ice crystals. The sun's altitude was around 10 degrees during the sequence.



At first there is a strong 22 degree halo with just a hint of extra brightness at the top.


A wisp of cirrus brightens as it moves through the tangent arc area.



A more extensive cloud of crystals of the right type reveals the distinctive shape.



A few minutes later the halo has faded and only the bright stub of the tangent arc is left.

At lower solar altitudes the upper tangent arc becomes convex to the sun. Here it just touches the top of the 22 degree halo and arches away on each side. (Solar altitude 10 degrees)

A few minutes later the 22 degree halo has faded but the upper tangent arc is still strong. The colour is noticeable here although not as strongly as in a bright sundog.