Colorful arcs in the sky

Jerry Tangren sent us this picture from Wenatchee, Washington on 26 March 2015 of amazing optical phenomena in the sky.

Halo_UpperTangentArc_Supralateral_Cirrostratus_JerryTangren_WenatcheeWA_26Mar2015

Jerry Tangren, Wenatchee, Washington, 26 Mar 2015

The 22-degree halo is the most common of these arcs, creating a full circle around the sun. However, the other arcs in this picture are much less common. Here we compare what we see in this picture to a generalized diagram from the incredibly educational Atmospheric Optics page: http://www.atoptics.co.uk.

alberta_halos

Labeled diagram of atmospheric optics from http://www.atoptics.co.uk

When we put Jerry’s picture within this context of this diagram, we are able to identify the other arcs as the upper tangent arc and the rare supralateral arc.

Halo_UpperTangentArc_Supralateral_Cirrostratus_JerryTangren_WenatcheeWA_26Mar2015_annotated

Jerry Tangren, Wenatchee, Washington, 26 Mar 2015 (annotated)

We know that these halos and arcs are created by the sunlight being bent through ice crystals. But which optical phenomenon occurs depends on how high the sun is above the horizon, what types of ice crystals are present in the cirrus clouds, and how the crystals are oriented relative to the sun.

For the 22-degree halo, the sunlight passes through hexagonal crystals, bending (refracting) twice as it passes through one face of the crystal and out the other. These crystals act like a prism, separating the light into the colors of the visible spectrum. This circular halo is relatively common because these crystals don’t have to have any particularly orientation for the sunlight to bend this way.

Prism

An example of a prism separating the light into the colors of the visible spectrum

Sitting atop the 22-degree halo is the upper tangent arc. Like the halo, this arc requires the sunlight to be bent through hexagonal crystals, but in this case, they must be columnar crystals (compared to plate-like crystals), and have to be oriented with their long axes nearly horizontal.

tangent

From http://www.atoptics.co.uk, a diagram showing how the sunlight must be bent through a columnar crystal to form an upper tangent arc (blue) as it’s long axes are oriented horizontally.

Note also that this arc is curved along its edges. The amount of curve depends on how high the sun is above the horizon. It flattens out the higher the sun is in the sky, although the limit to even see this upper tangent arc is about. 29 degrees above the horizon.

ts20

An example from http://www.atoptics.co.uk showing the curvature of the upper tangent arc when the sun is at an angle of 20 degrees above the horizon. Above 20 degrees, the arc flattens, while the closer the sun gets to the horizon, the more the arc bends.

Finally in the picture, we have the supralateral arc. This arc also requires columnar crystals, but instead of the sunlight entering the side of the crystal like for the upper tangent arc, it enters through the base of the crystal and out one of the prism faces. The shape of this arc also depends greatly on how close the sun is to the horizon.

So how rare is this? Well, a German group that studies halos did a study to determine how many days out of the year you could expect to see these different phenomena in the skies over Europe. Using 10 years of observations, they determined that the more common 22-degree halo could be seen 100 days out of the year, while the rare supralateral arc was only visible on about 4 days.

OpticalFrequency

From http://www.atoptics.co.uk, the relative frequencies of halo sightings in Europe by the German Halo Research Group.

The next question is, why are the supralateral arcs so rare? It has to do with the quality of the crystals (meaning no impurities along the edges to disrupt the bending of the light), the specific orientation of the crystals required, and how faint they are making them more difficult to see. For more details about the supralateral arc, as well as all atmospheric optical phenomena, check out the incredibly information page: http://www.atoptics.co.uk.

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