This week we travel much less distance, staying within our own atmosphere for a change. Although strictly not “astronomy”, allow me to introduce you to an area of sky observation to which I have only recently become acquainted, but which has rapidly become fascinating. It also helps that these observations can only be made in broad daylight, and (mostly) at any time of the year. The topic that has struck my interest is ice haloes.
We are all familiar with rainbows, caused by sunlight passing through falling rain droplets. The sunlight is refracted through the droplets as through a prism, with the colors of the spectrum separated at different angles as they emerge from the drops, producing the spread of color we observe in the sky. Rainbows occur infrequently, as they require both the unclouded presence of sunlight and falling rain, and with the sun at a low angle to the horizon. Rainbows are always of the same form, though particularly bright rainbows may be revealed as being double.
Ice haloes, on the other hand, require merely sunlight and high clouds in the upper atmosphere formed by ice crystals. There are a large variety of possible appearances for these haloes, dependent on the structure and orientation of the ice crystals involved, as well as the height of the sun over the horizon. The most common of the haloes occur very frequently, on average once or twice a week. The trick in seeing them is knowing where and when to look.
Water ice crystals formed in the upper atmosphere form hexagonal prisms – rods whose cross-sections and end faces are hexagons. The reason for the hexagonal form – which also dominates the patterns of snowflakes, though with very complicated and intricate substructures – lies in the chemical bonds of molecules of water, a topic I’ll defer today.
It is the fact that all of the ice particles in each upper atmosphere cloud have this prismatic form that causes the regular patterns of reflected or refracted light that we observe as haloes.
The most common halo is what is called the “22-degree halo”. This is an almost - colorless circle around the sun with a diameter of 22 degrees – if you stretch out your hand at arm’s length, put your thumb over the sun (please be careful to NOT LOOK DIRECTLY AT THE SUN), then the halo should lie just outside your pinky. This halo is formed when the hexagonal rods are poorly-aligned to each other, and light passes through one side of the rod and out through the second side away from where it entered the crystal (see the diagrams to visualize this). The 22-degree halo can also be observed around the Moon on a hazy evening.
The ice rods can come in a variety of lengths. Very short “rods” are more like hexagonal plates. These plates are much wider than they are thick, and so as they float in the air they tend to align so that their flat faces point up and down, with the narrow edges aligned horizontally. In this arrangement, light entering one edge and coming out two edges away will be directed horizontally. What we see in this case are “sundogs” – two areas to the left and right of the sun, about 22 degrees away, showing red, and often yellow, green and blue directed away from the sun. Look for these near morning or late afternoon on days with high thin clouds, when the sun is low in the sky.
Another beautiful effect produced by aligned plates occurs very close to sunrise or sunset. Light reflected off the upper or lower surfaces of the plates form sun pillars – rather bright beacons of light showing either above or below the sun
perpendicular to the horizon. The moon, or even city lights beyond the horizon can also cause pillars in this manner.
Perhaps the most stunning of the ice haloes is again caused by flat plates with the sun low in the sky. Unlike the other effects discussed so far, the circumzenithal arc is always centered not on the sun, but on the point in the sky directly overhead. The appearance is that of a small rainbow, but with the colors much more distinct than in a real rainbow, and with the peak of the arc pointing in the direction of the sun instead of away from it. Light enters the bottom flat face of the plate and exits through an edge. Because the conditions creating the circumzenithal arc are the same as those producing sundogs, the arc is commonly missed because it appears directly overhead, very far away in the sky from the sun, and from the much more noticeable sundogs.
Another beautiful spectral display is the circumhorizon arc. This phenomenon is basically the opposite of the circumzenithal arc – light enters the plate through the edge of the crystal, and exits through the bottom flat face. This arc can occur only with the sun very high in the sky – meaning that the arc is only visible in the summer – and runs in a huge line parallel to the horizon. Typically only sections of the arc are visible at any one time.
Longer rods will align with the long direction parallel to the ground. Light passing through in the same manner as before – through two faces of the long edge of the crystal – will now be deflected either “up” or “down” in the sky and form upper and lower “tangential arcs” that touch the 22-degree halo and then form wings that branch away from the halo. Because these arcs also rely upon horizontally-aligned crystals, they are again best seen with the sun low in the sky.
The last of the common ice haloes is the parhelic circle. This is a white circle running parallel to the horizon at the same height as the sun. When complete (which is rare), it has the appearance of a vast ring with the sun as its jewel. This is formed from the reflection of sunlight off both plates and rods aligned horizontally off various edge and end faces.
In addition to the 7 halos discussed here, there are a large variety of relatively rare haloes of very different shapes and sizes, some colored, others colorless. These require much tighter crystal alignments, or may be generally faint and therefore go unnoticed.