Halos can be as colorful as rainbows, appear all over the sky, come in lots of shapes (including circles, ovals, arcs, spots, pillars, and crosses), and are almost as unknown to the public as their arch-rivals are famous.
In January 1987 I was one more of the ignorant masses, happily spending my summer vacation by the sea, when I was struck by a strange rainbow in the sky. A large colored circle surrounded the sun, cast against a backdrop of thin clouds. At the time I didn't notice that the color sequence was the reverse of the rainbow, with the sharp inner-edge red, but I realized it was not the familiar arch. It wasn't raining, it appeared in the direction of the sun, and it was a complete circle. What could it be? Inspired by some goddess, I took this photograph with my pocket camera. It was the only one that I took and inevitably could only cover part of the circle, but it turned out quite good considering the lack of exposure control and that I didn't take the precaution of covering the sun. That was my first Halo!
Halos are created by reflections and/or refractions on hexagonal ice-crystals, instead of water drops. Although they are more common in cold weather (some of the best photographs are from the South Pole), halo-producing cirrus clouds can be present in warm weather (by the same token, rainbows are less common in winter). I know it from experience: besides my first halo, which I photographed with the sun close to the zenith during the austral summer (Punta del Este, Uruguay, latitude 35 degrees south), I have also seen these displays in Hot South Texas. But most people remain oblivious to these extraordinary displays. One reason is that the most common halos happen in the direction of the sun, a region of the sky we instinctively try to avoid; and the bright background of clouds and sky provides less contrast than the dark frame of the rainbow. Another reason is that we tend to be blind to things we are not aware that exist: it really pays off to learn where to look and what to expect.
The most common halos are the Sun Dogs (also called mock suns or false suns): bright multi-colored patches of light on both sides of the sun (to find them extend from the sun one hand at arm length). Second in prevalence is the 22 degree halo (of previous fame), which passes through the sun dogs and can have upper and lower tangents arcs or be circumscribed by another odd shaped halo. Other less common halos include a 46 degree halo also centered at the sun, two very colorful (even more than a rainbow) arcs centered on the zenith, a horizontal white circle through the sun, vertical pillars above the sun (or an artificial light), rare crosses opposite to the sun, the very bright sub-sun often seen from airplanes, and many minor or rarer arcs and spots, too many to enumerate here.
Colored halos are formed by refraction in the crystals and are generally just slightly polarized. With a polarizing filter oriented in a radial direction, the 46-degree halo and the beautiful circumzenithal arc are seen more clearly against the sky (which is polarized perpendicularly). On the other hand, the most notable effect on the 22-degree halo and sun dogs is a small shift in position: the inner red edge is a tad closer to the sun when the polarizing filter is radial. The reason is that each polarization causes a different halo due to the birefringence of ice.
White halos are produced mainly by reflection and they can show stronger polarization, generally tangential to the sun. However, if most of the light comes from total internal reflection, no polarization is seen. For example, the sub-sun is often strongly polarized in the horizontal direction but, for some sun altitudes, it becomes almost unpolarized.
Why waste time checking the polarization of these halos? Because it is there. Besides, in the case of the more rare halos their origin may be controversial and a polarization measurement can pinpoint the correct explanation (for some halos there is still NO explanation). But, for sainthood sake (you want your halo, don't you), first take a few pictures, then note its position and size in the sky, and finally get your polarizer out.
Update 9/9/1999: Nasa is looking for the 22 degree halo in the ice cover of Jupiter's moon Europa, using images from the space probe Galileo. If it finds the halo, it would be proof that the ice is in the form of hexagonal crystals, which only form at temperatures above 170K. Scientist suspect that a huge liquid ocean (biggest of the solar system) lies beneath Europa's ice crust, heated by tidal forces from Jupiter. If that were true, it would be an excellent place to search for extraterrestrial life.
The Specter of the Brocken was a rather mysterious phenomena. Somebody would laboriously climb a mountain and break through the clouds into the bright sunlight. Then he would turn around and be confronted by a giant with multi-colored rings around his head. The spectacle was named the specter of the Brocken after the peak in the Harz Mountains of Germany, where it was often seen. The giant figure was the shadow of the climber cast over the fog, who often would exaggerate its size by miscalculating its distance. But the colored rings around the head, the Glory, proved much more difficult to explain. Nowadays the glory is very often seen from airplanes, surrounding its shadow on the clouds below (however, if the airplane is high above the clouds, only the glory remains).
The physical origin of the glory was an optical riddle until the seventies, when it was calculated correctly by scattering theory. The small water drops of clouds and fog produce it; however, no intuitive physical explanation is available. In part the glory comes from light guided by the interface of water and air (surface waves) and in part from light that undergoes ten (!) internal reflections (a tenth rainbow?). If you look at a single drop, you will see a ring of light shining on its periphery.
The polarization of the glory is extremely unusual. The colored rings are radially polarized, contrary to a rainbow. On the other hand, the white region close to the center is tangentially polarized. With a linear polarizer a distinctive pattern appears, formed by sections of the colored rings plus dark triangles in the center region pointing towards the center.
It is worth to look for the glory when traveling by plane as it is quite common. In fact, at one time some pilots used it to find the position of the sun when it was behind them and there was no visible shadow ("shooting the glory"). Interestingly, the pilot will see the glory centered on the front of the aircraft shadow (if visible), while a passenger on the last seat will see it centered towards the tail of the shadow. Many times you will see a long narrow shadow starting at the glory and going backwards. It is the shadow of the contrail, the vapor trail left by the jet turbines.
Karl Grobl sent me this picture that he took over the Grand Canyon (copyright Karl Grobl Photography). Can you guess where on the airplane was Karl sitting? Notice that at least 3 glory rings can be seen (full image), indicating that the water drops were of very uniform size.
Zodiacal Lights can only be seen in very dark moonless nights, very far from city lights (sorry, no hope of zodiac enlightenment in Las Vegas). It is best seen between two and three hours (depending of your latitude) after sunset or before sunrise. Its appearance is similar to the Milky Way, but it is not fixed with respect to the stars. Its origin is light scattered by interplanetary dust. It is only polarized to a small degree (15%), in a direction tangential to the sun (the Milky Way is unpolarized).
Diffraction Coronas frequently appear as an aureole around the moon, the sun, and bright lights when partially blocked by clouds, mist, or dust particles. If these are of uniform size, distinct rings appear, colored from blue on the inside to red on the outside. The size of the corona, as the glory, is only a few degrees in diameter. Unfortunately for this webmaster, they are unpolarized.
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