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 I describe here, take a minute and see if you too
start noticing these features with much higher frequency.
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 (opposite direction from the sun). 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 (see the third photo above), and Alexander's
dark band (clearly evident in the second photo). The diagram (click it for
more detail) illustrates how the primary, secondary, and Alexander's dark
band are created. The Lynch and Livingston reference (see below) includes
detailed explanations of all of these features and much more with excellent
diagrams and photos. 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 as in the leftmost
photo of row 2 (WxOpti01b_01).
Also common are perhelia, often called sun dogs which 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 vs. 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 (below) 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.
*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.
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. As
in photo WxOpti03b_08, 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
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. The iridescent cloud photos shown at the top of this page are some of
the best 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
Minnaert, M.J.G., 1954: The Nature of Light and Colour
in the Open Air.Dover Publications.
Lynch, D.K. and W. Livingston, 1995: Color and Light in
Nature. Cambridge University Press.
Bohren, C.F., 1991: What Light Through Yonder Window
Breaks. John Wiley and Sons, Inc.
Greenler, R., 1980: Rainbows, Halos, and Glories.
Cambridge University Press.
Weather Gallery (Page 3 of 6): Optical Phenomenon
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