 Locating
Rainbows
A low Sun produces
the largest rainbows, which also tend to be red rich in
color. The long pathway through the lower atmosphere
scatters the shorter wavelengths of blues and greens,
leaving the remaining light proportionately higher in
yellows and reds. This giant orange slice flavored the
day's end in Istanbul, Turkey, on August 4, 2005. Ozan
Kilic
To precisely locate a rainbow, stand with your back to
the Sun and find the head of your shadow—this is the
antisolar point and the center point of the rainbow’s
arc. Stretch out your arm and point your index finger at
your head’s shadow. Hold out your thumb at an
approximate 42° angle (half a right angle) and pivot
your wrist while keeping your index finger aimed at your
head. Your thumb traces out the direction of the arc;
take notice where the rainbow
meets the horizon.
 Observing
Halos
Complex
choreography bedazzles the sky with a luminary cast over
Turku, Finland. From top to bottom: arching upward:
circumzenithal arc; arching downward from the same
point: a supralateral arc; the upper portion of the
bright disk in the middle is a Parry arc; the lower
gull-winged portion is an upper tangent arc; a 22° halo
with a pair of parhelia on both sides, with a portion of
the parhelic circle extending away. Pekka Parviainen.
Halos happen when light shines through ice crystals in
the air. This condition may be visible in simple streaks
of cirrus or thick banks of cirrostratus, or quite
invisible in a sparse veil. In addition, the Sun must be
shining through them in particular angles in order to
create the various designs in the sky. While this can
happen at any time, we are more apt to see halos and
their kin when the Sun is higher in the sky, both daily
and seasonally. The full Moon also creates its own halo
gallery in similar conditions, so be aware for their
nighttime exhibitions as well.
To see solar halos well, you must first block the Sun’s
glare with your hand or another object. While your hand
is in the air, you can also get a handle (so to speak)
on the caliber of the event. Stretching out your arm,
placing your thumb over the Sun, and spreading your
fingers spans 20-25° of angular measurement. If the halo
under scrutiny is the most common one at 22°, you’ve
nailed it at your fingertips.
If you see one halo, there could be others. Scan the
entire sky, including behind you, to discover if there
are any other accompanying arcs, circles or spots.
 Locating
Mirages
One of the most common mirages is the seeming appearance
of water on a highway as light from the sky is bent low
on its way to our eyes. In this view, the dark asphalt
of the recently repaved Highway 60 in New Mexico warms
the air immediately above it, creating the mirage. Seven
(or eight?) varying images of the vehicle precede it;
some merged, some inverted, some not. Harald Edens
The mirage is a true thing: a distorted image of real
objects caused by light bending through the air.
Mirages are not dependent on warm temperatures per se,
but on the rate of temperature change with height or the
difference in temperatures between adjacent or
alternating layers of air. The greater the rate of
change, the greater the light bends on its way to our
eyes.
Mirages appear whenever the momentary conditions allow.
Surprisingly, such conditions are quite widespread and
happen fairly frequently. The trick is knowing where and
when to look. Compared to the expanse of the entire sky,
mirages are tiny. The most likely places for look for
them are near the horizon where…
a strongly heated flat surface radiates its warmth to
the immediately overlying still air, such as above the
desert floor or the dark pavement of a road during the
day, or over a calm lake at night.
a very cold surface cools the air above it, as in arctic
regions, or is overlain with much warmer air, as when
lake or sea ice thaws in springtime.
a strong temperature inversion (cooler air below warmer
air) forms. This is evident where smoke stays at the
ground level or stops rising at a certain height,
revealing an inversion aloft.
 Observing
Auroras
From vaunted prominence at the top of the world comes
another source of glorious and peerless magnificence.
Twisting and undulating wisps, bands, streaks and
curtains release electrical energy and spirited
spectacle on our heads. Auroral rays appearing to
radiate away from a point is referred to as a corona.
Jan Curtis
The sky must be clear and dark. The naked eye is the
best equipment since binoculars and telescopes cannot
improve the view on such nebulous structures.
At high latitudes, the aurora is visible nearly every
night, at least faintly even when the solar activity is
calm. But since summer nights do not get dark, the best
times are during the winter, when the weather also tends
to be clear and cold.
Because of the geometric relationship between the Earth
and Sun, displays that can be seen from mid- and lower
latitudes are more likely to occur during the months of
March-April and September-October when nights and days
are roughly equal in length. Plan to search the skies
between 11 pm and 2 am as the most active and brilliant
displays usually occur around midnight.
©
Tim Herd |
