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In the Clouds Photography
Optical phenomenon in the atmosphere
The most well-recognized atmospheric optical phenomenon is a rainbow but water drops and ice crystals produce a variety of optical effects; many of which occur regularly but go unnoticed because of our natural tendency not to look into the glare of the sun. Next time you are outdoors under some of the conditions described here, take a minute and see if you too start noticing these features more regularly.
Rainbow
Two ingredients are absolutely necessary for rainbow formation: light and water drops. The light can actually be from moonlight as well as sunlight and produces a faint colorless moonbow, because the light from the moon is far dimmer than direct sunlight. The most visible rainbow is the primary bow and has a radius of 42° centered on the anti-solar point Often a secondary bow is visible at 51° thereby giving a double-rainbow. Believe it or not a third rainbow is possible and has been observed. It would be found at 137° from the anti-solar point or 43° away from the sun and would be much dimmer and broader than either the primary or secondary. I have yet to see one in person or see a photograph of one but I hope to photo one myself. Bear in mind that it will be quite difficult to spot because of its dimness, looking toward the sun, and the background may not be dark enough to provide the proper contrast.
Other named rainbow phenomenon include less frequently visible
supernumerary bows, and Alexander's
dark band. The diagram below illustrates how the primary, secondary,
and Alexander's dark band are created by reflections within water drops.
The Lynch and Livingston reference (see sidebar) includes
detailed explanations of all of these features and much more with excellent
diagrams and photos throughout. I highly recommend the book to all.
Halos, perhelia, glory
Besides rainbows, the sky is filled with other colorful arches
and optical effects. Halos often appear around
the sun or moon and can have fringes of washed or metallic colors. The most
common halo is 22°*
around the sun or moon.
Also common are perhelia, often called sun
dogs that are found to the left and
right of the sun at 22° (same elevation as the sun). These can be colored
or just bright colorless regions on one or both sides of the sun sometimes
extending horizontally away from the sun.
Both halos and sun dogs are produced by hexagon shaped
ice crystals typical in thin cirrus clouds. The main contributor to whether
a halo appears versus perhelia is the orientation of the ice crystals. When
the crystals are randomly oriented a halo results whereas when the crystals
tend to lie flat horizontally, perhelia result. Again, refer to the
Lynch and Livingston reference (sidebar) for detailed diagrams and discussions
of many more halo and arch positions including more rarely observed
9°, 18°, 20°, 24°, 46° halos along with parhelic circle,
circumhorizontal, and circumzenithal arcs.
A glory, pilot's bow, or Brocken bow is now a very common site for anyone who flies over a uniform cloud deck with sunny skies above. A glory consists of colored rings centered on the anti-solar point usually within 10°. Mountain climbers were probably the first to see this feature with their shadows in the center and glorious colored rings surrounding their heads. Such spectacles are called Brocken spectre. The rings are produced by light that has traversed the periphery of cloud droplets and scattered back to the eye.
Iridescent clouds (irisation)
Perhaps more common than all of the above are coronae and iridescent clouds. Both are due to diffraction by tiny cloud drops. When the drops have very uniform size, the colors tend to be brighter and pure. But, when the drops have varying sizes, the colors are washed and tend toward white. This iridescent cloud photo shows one of the best displays I have ever witnessed but isolated patches of iridescent clouds are very common if you just take the time to look toward the sun when thin clouds are present. Blocking the sun with your hand or other object (tree, lamppost, etc.) as well as wearing sunglasses can aid in spotting this phenomenon regularly.
Sponsored links:
Suggested reading & related info:
The Nature of Light & Colour in the Open Air
Color and Light in Nature
What Light Through Yonder Window Breaks?
Rainbows, Halos, and Glories
*Note: An easy way to estimate if a halo or sun dog
is 22° from the sun is to hold up your arm outstretched and spread your thumb
and pinky-finger; while placing your thumb over the sun. The angular measure
between your thumb and pinky is 22°. This general rule works extremely well
for all people, adults and kids alike since kids have short arms but small hands.
Try it sometime and if your thumb covers the sun while your pinky touches the
halo or sun dog, then you know it is a 22° halo.