Physics Photo of the Week
November 9, 2012
Moon halo with tangent arc
Notice
the rainbow colors in this spectacular lunar halo taken in
October, 2010. Lunar (and solar) halos are fairly
frequent sights caused by ice crystals present in cirrus
clouds in the atmosphere. The amazing features of this
halo are not only the colors (similar to the colors produced
by glass prisms). This halo also produces the "upper
tangent arc" - the larger radius arc that is tangent to the
main halo at the top - and the "circumhorizontal arc" -
short sections of tilted circles emanating from beyond the
halo at about "2:30" and "9:00" positions on the circle.
A Sun halo has been described before (PPOW
for April 6, 2012). The
main circle is caused by the refraction of hexagonal
pencil-like ice crystals. The ice
crystals are gently falling in the air. The friction
with the air causes objects to orient themselves to maximize
the drag. Leaves and confetti fall flat, not edge
on. The upper tangential arc
is also caused by the
same hexagonal pencil-like crystals. In both the main
circle and the upper tangential arc, moonlight (or sunlight)
enters one face of the hexagon, and emerges two facets from
where it entered. See the diagram below. The
long axes of the needle-like ice crystals are horizontal,
but randomly directed in the horizontal plane. For an
ice crystal in the main halo to contribute to the halo, the
orientation of the long axis of the crystal is 90 degrees to
the Moon's ray (or Sun's) and the refraction for ice is 22
deg from the original direction. For a crystal to
contribute to the upper tangential arc, the ray of moonlight
(or sunlight) enters the face of the crystal obliquely, not
perpendicular to the long axis of the crystal, thus
resulting in a much more complicated halo. The drawing
actually shows the oblique incidence needed for the upper
tangential arc. The upper tangential arc is more
completely described at the following link: http://www.atoptics.co.u/halo/column.htm
Finally, the faint arcs leading out of the halo at the
2:30 and 9:00 positions are the result of refraction of
hexagonal plate crystals that resemble small hexagonal disks
rather than pencils. Because of aerodynamics the
plate-like crystals fall with a horizontal
orientation. If the Moon (or Sun) is close to the
horizon, these "circumhorizontal" arcs produce a sundog (or
parhelia - see
PPOW for Feb. 8, 2008). Because the Moon's
elevation is considerably higher than the horizon in today's
photo, the parhelia lie outside the 22 degree halo.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
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