six distinct colors in the spectrum: red, orange, yellow,
green, blue, and violet. However, a mixture of these
colors is also present.
ATMOSPHERIC OPTICAL
PHENOMENA
LEARNING OBJECTIVE: Identify the
characteristics
of
atmospheric
optical
phenomena
(halos,
coronas,
rainbows,
fogbows, mirages, looming, scintillation and
crepuscular rays).
ATMOSPHERIC LAWS
Atmospheric
optical
phenomena
are
those
phenomena of the atmosphere that can be explained in
terms of optical laws. Some of the atmospheric
elements, such as moisture, serve as a prism to break a
light source down into its various component colors.
The resulting phenomena can be spectacular as well as
deceptive.
Halos
A halo is a luminous ring around the Sun or Moon.
When it appears around the Sun, it is a solar halo; when
it forms around the Moon, it is a lunar halo. It usually
appears whitish (caused by reflection), but it may show
the spectral colors, from refraction (red, orange, yellow,
green, blue, indigo, and violet) with the red ring on the
inside and the violet ring on the outside. The sky is
darker inside the ring than outside. Halos are formed by
refraction of light as it passes through ice crystals. This
means that halos are almost exclusively associated with
cirriform clouds. Refraction of light means that the
light passes through prisms; in this case, ice crystals act
as prisms. Some reflection of light also takes place.
Halos appear in various sizes, but the most
common size is the small 22-degree halo. The size of
the halo can be determined visually with ease.
Technically, the radius of the 22-degree halo subtends
an arc of 22°. This simply means that the angle
measured from the observation point between the
luminous body and the ring is 22°. Halos of other sizes
are formed in the same manner.
Coronas
A corona is a luminous ring surrounding the Sun
(solar) or Moon (lunar) and is formed by diffraction of
light by water droplets. It may vary greatly in size but is
usually smaller than a halo. All the spectral colors may
be visible, with red on the outside, but frequently the
inner colors are not visible. Sometimes the spectral
colors or portions of them are repeated several times
and
are
somewhat
irregularly
distributed.
This
phenomenon is called iridescence.
Rainbows
The rainbow is a circular arc seen opposite the Sun,
usually exhibiting all the primary colors, with red on
the outside. Diffraction, refraction, and reflection of
light cause it from raindrops or spray, often with a
secondary bow outside the primary one with the colors
reversed.
Fogbows
A fogbow is a whitish circular arc seen opposite the
Sun in fog. Its outer margin has a reddish tinge; its inner
margin has a bluish tinge; and the middle of the band is
white. An additional bow, with the colors reversed,
sometimes appears inside the first.
Mirages
Mirages are images of objects that are made to
appear displaced from their normal positions because
of refraction. These images may be only a partial image
of the object, and they may appear in either an upright
or
an
inverted
position,
depending
upon
the
atmospheric condition that exists at the time of
observation. Mirages occur when adjacent layers of air
have vastly different densities because of great
temperature differences. Whether these layers exist
side by side and horizontally or vertically determines
the type of mirage.
Mirages are often seen in desert areas where air
near the surface becomes very hot. Cool air overlies this
hot layer resulting in a large difference in the densities
of the two layers. Three types of mirages result from the
refraction of light rays through layers of air with vastly
different densities.
INFERIOR MIRAGE.—The inferior mirage, the
most common of the three, appears as a mirrored image
below the object being viewed by the observer. In this
case, you can associate the word inferior with beneath
or below.
SUPERIOR MIRAGE.—In the superior mirage,
the mirrored image appears above the object being
viewed. In this case, associate the word superior with
above or over.
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