Halos, arcs, and bows…oh my!
The term “landscape photography” is a misnomer in some ways. After all, many of the most interesting and unique landscape shots are those in which something unique or interesting is happening in the sky: a vibrant sunset bathing the land in a golden glow, an ominous storm looming on the horizon, or a terrestrial scene backed by a sky awash in stars. A colorful sky, interesting clouds, or a stray meteor can single-handedly liven up otherwise passé landscapes. As a longtime astronomy educator, I have a habit of looking up…a habit that often pays photographic dividends. I had an astronomy professor in college who would often remark that “most people just don’t look up”, a sad but true (save for perhaps an occasional glace at the clouds on a stormy day) acknowledgement of just how little attention most of us pay to the sky above us.
One of the things that we miss by not looking up is a myriad of features that result from the interaction of sunlight with water droplets or ice crystals in our atmosphere. These features lack a catchy collective name, but scientifically are often referred to as atmospheric optical phenomena. Perhaps the most frequently observed example is the humble rainbow:
The colorful band of light in a rainbow results from a process known as dispersion. When sunlight passes through water droplets in the Earth’s atmosphere, different colors of light are refracted (or “bent”) by different amounts as they pass through the droplet, thus spreading out the colors that make up “white” sunlight into a rainbow. The water droplets don’t have to be rain; sea spray or the mist of a waterfall can do the trick as well. The geometry of dispersion is such that the center of a rainbow’s curvature will always be directly opposite the Sun. Consequently, the largest and grandest rainbows are seen near sunrise or sunset (see photo below) when our star is low on the horizon. In contrast, when the Sun is higher in the sky, only a small portion of the arc will be visible (see photo above).
The light source for a rainbow need not be the Sun either. Light from the Moon can also be dispersed through water droplets, with a similar result. The caveat? Even a bright full moon is about 400,000 fainter than the Sun, so the resultant moonbow is much fainter than a rainbow and the colors not bright enough to trigger the color-seeing cones in our eyes. Thus, a moonbow appears monochromatic. I’ve only ever witnessed this eerie phenomenon once: on a damp and muggy evening on the north coast of New Zealand’s South Island, and unfortunately did not have the wherewithal to capture a photo at the time.
Moving beyond water droplets, the possibilities when sunlight interacts with tiny ice crystals in our atmosphere (such as those that comprise high altitude clouds like cirrus and cirrostratus) are myriad. Depending on the shape, size, and orientation of said crystals, the height of the clouds, and the location of the Sun, a wide range of atmospheric phenomena can result when we see the Sun through these high-altitude icy clouds.
One of the most common is the 22° halo, a ring of light that encircles the Sun (or the Moon) at a radius of (you guessed it) 22 degrees:
These halos can be a little difficult to spot (and photograph…) due to the Sun’s glare, but in most locations they are, statistically speaking, much more common than rainbows. Like rainbows, these halos are caused by dispersion as sunlight passes through tiny ice crystals in the atmosphere. If you look VERY carefully (such as in the photo above), you can see that the inner edge of the halo is reddish while the outer edge is bluish.
The name “22° halo” suggests that halos can appear at other distances from the Sun as well, which is indeed the case. Another member of the halo family is the circumhorizontal arc, a halo that takes the form of a brightly colored band circling the horizon at a radius of 46° from the Sun. Unlike 22° halos, circumhorizontal arcs are rarely visible in their entirety. Instead, you’ll usually only see one or two small fragments in areas of the sky where the background consists of the correct type and height of cloud. These fragments look like pieces of a rainbow oriented parallel to the horizon:
In order to see a circumhorizontal arc, the Sun needs to be at least 58° above the horizon. This means that, at least for us here in the mid-Northern latitudes, they are only visible in summertime. The rest of the year the Sun simply doesn’t get high enough in the sky to allow them to be seen, even when the right types of ice crystals are present. (Further north, close to the poles, they can’t be seen at all!) Indeed, a quick review of my photo archives shows that all the photos I have of this phenomenon were taken in late June or July.
Circumhorizontal arcs bear some resemblance to another phenomenon known as cloud iridescence. Unlike the features we’ve discussed so far, cloud iridescence is the result of diffraction (as opposed to refraction and dispersion), which is when light waves are bent around objects. Iridescence occurs at the edges of very thin clouds that are made of very similar sized water droplets or ice crystals. If the cloud layer is thin and uniform enough, light waves bent around these particles interfere with each other, producing a spectrum of colors. Without getting into the nitty gritty details, the colors that you see in an iridescent cloud are more akin to the colors that you sometimes see in soap bubbles or a pool of oil.
Despite their similar appearance, cloud iridescence and circumhorizontal arcs are relatively easy to distinguish from one another. Fragments of circumhorizontal arcs always appear (you guessed it) horizontal relative to the horizon and will always have a bright red band on top. In contrast, cloud iridescence can appear in all sorts of shapes and patterns, and you’ll often see the spectrum of colors repeating themselves multiple times as well, as seen in the photos above.
Like rainbows, many other atmospheric phenomena occur only when the Sun is close to the horizon, just before sunset or just after sunrise. Sun pillars, like those in the photos below, are the result of sunlight being reflected off the surface of flat, hexagonal-shaped, ice crystals and redirected back to the observer:
Once in a great while, conditions will be just right and you’ll get a whole truckload of these phenomena all at once, such as on the evening I took the photo below on a recent neighborhood walk as the Sun set over the Cascade range:
Sundogs and tangent arcs will have to wait until another day. Until then, keep your eyes on the sky. Chances are you’ll spot a nalo, arc, or bow before too long!