According to Wikipedia: A sun dog or sundog (scientific name parhelion, plural parhelia, i.e. "beside the sun") is a common bright circular spot on a solar halo. It is an atmospheric optical phenomenon primarily associated with the reflection or refraction of sunlight by small ice crystals making up cirrus or cirrostratus clouds. Two or more sundogs can often be seen on opposite sides of the sun simultaneously.
So, based on this, the photo on the left is actually a photo of sundogs, while the one on the right is merely the halo. (Photos are also from the Wikipedia entry on sundogs.) While I've seen these halos plenty of times, I've never seen the bright spots, or actual sundogs.
Another interesting atmospheric phenomenon that I have looked for for years is iridescent clouds. I came across these in a book entitled "The Color of Nature," by Pat Murphy and Paul Doherty. According to them, "Most clouds look white because they are made of water drops or ice crystals that scatter all colors of light in all directions. But occasionally, when conditions are just right, thin, high clouds will scatter colored light, creating beautiful bands of color. These clouds, known as iridescent, or opalescent, clouds, are made up of uniform drops of water, each of which is about the same size or smaller than a wavelength of red light. The drops scatter light of different wavelengths at different angles, creating the colors shown here. Bands of color may appear at the edge of a cloud. When a cloud with small, uniform water drops passes in from of the sun, the colors circle the sun, a phenomenon known as a corona."
The photos (included here) amazed me and I started my search. I asked other naturalists I knew if they had ever seen these; the answer was always no. Then lo! and behold! two summers ago (2006), while headed home from our morning walk (the dog and I), I looked up and saw iridescence in the clouds behind a tall white pine. I was stunned...amazed...in awe. And the next morning, I saw the same. It happened several times that summer. Hm...something must not be right, thought I. Afterall, what are the odds that I never see the things and now suddenly they are almost as common as fleas on a dog? I did some "experimenting" - that is to say, I took my glasses off the next time I saw the shimmer of colors in the clouds. And the colors disappeared. What a disappointment - it seems the various treatments on my lenses were affecting the wavelengths of light reaching my eyes, enhancing the colors in the clouds.
Does this mean I didn't really see the iridescence after all? HM. I have new glasses now, so my search continues.