Now and then you see some cool, somewhat rare things in the sky if you take a minute to look up! Ramona Edwards – one of our faithful citizen photographers in Huntsville – is always looking for that opportunity, and she captured a neat 22º halo around the Sun this morning and something that you don’t see very often: an upper tangent arc.
A tangent is ‘a straight line or plane that touches a curve or curved surface at a point, but if extended does not cross it at that point.’
The upper tangent arc does just that: touching the top of the 22º halo but not really intersecting it.
Ramona also captured a sundog in the thin cirrus clouds overhead this morning (official name: ‘parhelion’).
Sundogs, halos, and arcs are a form of atmospheric optics that occur when the sun’s rays pass through ice crystals high in the atmosphere. The ice crystals indicate that moisture is present in the atmosphere, well above the freezing layer.
In the case of sundogs, the ice crystals are oriented in such a way that their large faces are parallel with the ground. This allows sunlight to pass through one of the edges, when it then refracts (or bends). The refraction is occurs at 60 degrees from the edge of the crystal, or 20 degrees from the original path of the sun’s ray if it hadn’t entered the crystal.
The sunlight travels through the crystal and exits out of a different edge, refracting once again. Red light refracts a little more than the blue light, so you see the reddish hue closer towards the sun.
It takes more than one ice crystal to produce a sundog. In fact, the phenomenon that you see occurs because many crystals are present and their sunlight is being directed back to your eyes.
Sun halos and sun dogs can be seen when thin, wispy cirrus clouds are present in the sky. These clouds are often present a few days before an on-coming storm system, or even behind a departing storm system.
If you were able to capture this beautiful phenomena, send us a photo using the link below.