Atmospheric Optics
Bend, Bounce, and Scatter
Why is the sky blue?
First, remember that visible light is made up
of a spectrum of colors.
We see objects as a certain color because
that color is either reflected from an
object or scattered by the object
(scattered means it was absorbed and
then re-radiated)
RED GREEN BLUE
The atmosphere is made up of mostly nitrogen
and oxygen. They are selective scatterers,
meaning they scatter short wavelengths best
(violet, blue, and green). Our eyes are most
sensitive to blue light, so the sky appears blue to
us!
Why do clouds look white and
sometimes dark?
Water vapor (clouds) scatter all wavelengths
equally. The result is white.
When clouds are thick (like thunderclouds) they
absorb much of the light. Water drops also tend
to absorb light. The result is a darker cloud.
Why are sunsets red?
The Green Flash
•
•
Green flash is an atmospheric phenomenon observed occassionally at sunset. Remember
at sunset that the light travels through a much greater amount of atmosphere—this bends
the light from the setting sun so that we see the sun for a short while after it has actually
set. Blule light bends the most so we should see some blue light at the top of the setting
sun. However because the blue light is scattered out the most very little reaches us and
we see green light instead.
Usually the green light is too faint to see with the human eye, but under certain
conditions (hot surface air or an upper level inversion) it can be seen for about a second
or so. (polar regions can see it for longer
Particles in the
sky (dust,
water droplets,
or haze)
scatter light in
their path
making that
region appear
bright with
rays
Crepuscular
Rays
(Jacobs Ladder)
Inferior Mirage: It looks like there’s
water on the road
A layer of very warm air near the ground refracts (bends) light
from the sky that is headed down toward the ground to our eyes.
Our mind interprets the light rays reaching our eyes as having
come along a straight path. Thus, we see that patch of sky and
cloud "on the ground" and interpret the image as a surface "pool
of water."
The layer of hot air above the ground causes the inferior
mirage. The key is not what the temperature is, but what
the temperature difference is between the layer at the
ground and the air above it. Highway mirages are as
common over dry pavement on sunny winter days as
during the summer months.
Measurements of air temperatures within 1cm of the
pavement have been recorded 20 to 30 F degrees higher
than the air temperature above that layer!
Van’s
refraction
Any object can be seen that is located at or beyond the mirage's
apparent location. If you look closely, you can often see details
such as vehicles in the highway mirage. Images are often
inverted in the mirage, thus giving the appearance of the object
being reflected in a pool of water.
How is a
rainbow
formed?
Remember that light is dispersed by a
prism..
Sun’s rays
I’ve only
drawn in the
two
extremes:
violet and
red. All the
other colors
are in
between
these two.
Each raindrop acts like a prism and separates the
colors.
We see only one color for each
drop because of it’s exit angle. The
other colors from that drop are
either too high or too low. They
don’t hit our eyes.
Double Rainbow. Notice the secondary bow has reversed
color order.
Halo
around the
sun
Light is
refracted by tiny
suspended ice
crystals
Sundogs
(parhelia)
Halos are formed with the ice crystals
are randomly oriented. Sundogs are
formed when the crystals are oriented
in the horizontal plane as shown here.
Tangent
arc
Formed when
large pencilshaped ice
crystals are
falling
parallel to the
ground
Sun Pillars are
formed when
sunlight reflects
(bounces) off ice
crystals. They
may extend
upward or
downward from
the sun.
Glitter Path
Due to diffraction
(bending around
an objects) from
tiny cloud droplets.
Colors are
sometimes seen
when drops are of
uniform size
(different
wavelengths are
bent different
amounts)
Corona
Stay Curious!
Sources:
http://www.islandnet.com/~see/weather/elements/infmrg
e.htm
http://www.weather-photography.com/index.php
http://australiasevereweather.com/photography/index.ht
ml
http://www.env.leeds.ac.uk/envi1280/lecture_notes_car
slaw2/sld001.htm