Rare Halo Display Over Alpharetta, Georgia
November 12, 2009
The photo above shows an outstanding ice halo display with both bright and very rare arcs. It was taken near Alpharetta, Georgia on December 5, 2008 with the sun 30 degrees above the horizon.
Most prominent are the brilliant upper tangent arc (strictly speaking a circumscribed halo because the sun is just higher than the 29 degrees when the name changes) and a suncave Parry arc (see the Earth Science Picture of the day for November 2, 2009). The upper tangent arc is so named because it lies above the Sun and is tangent to the 22 degree halo, the familiar ring around the sun also in the image. The Parry arc is much rarer and is named after the polar explorer William Edward Parry who first described it in 1820. Both halos are produced by similar ray paths through six sided, column shaped ice crystals drifting with their long axis oriented nearly horizontally. Those making the tangent arc have all rotational positions about the long axis. Those making the Parry arcs do not rotate but are instead most improbably oriented with two prism side faces horizontal. The different crystal orientations dramatically change the halo produced. Another crystal orientation and shape, plate crystals, made the bright sundog to the right.
Two very rare halo arcs indeed are the choice gems of this display. Curving upwards and outwards from the 22 degree halo to touch the Parry arc is a faint and elusive upper Lowitz arc, long sought since Lowitz first drew it in 1790 and photographed for the first time only a few years ago. If that is not enough, an even rarer helic arc produced by reflection from the side faces of the Parry oriented crystals extends up from the sun, crossing the 22 degree and upper tangent arcs.The arcs are all labelled on the matching HaloSim ray tracing simulation.
Always be sure to protect your eyes when looking for halos.