- describe the nature of Rainbows, Sun Dogs and Light Pillars
- describe the nature of Noctilucent clouds
- describe the nature of Zodiacal light
There are many other interesting effects that exist
within our atmosphere. The first that we will discuss involves water
vapour in the sky reflecting and refracting sunlight. This results in
several phenomena such as Rainbows, Sun Dogs and Light Pillars.
Rainbow after rain shower
occur as the light from the Sun hits droplets of moisture in the sky
and reflect and refract back to our eyes in the various colours of the
spectrum. As the light hits the rain droplets, it passes through the
front side of the water droplet and refracts or bends slightly. Then
the light hits the back of the droplet and some of the light reflects
back towards the observer. Different wavelengths of light refract at
slightly different angles. In other words, red light bends at a
slightly different angle than yellow or violet light. This creates the
arc of colours that makes up the rainbow.
The light from the Sun bounces off water droplets in the sky at
different angles for each wavelength, or colour, of light.
Water droplets are needed for this effect to occur
so rainbows are most often seen after a rainstorm when there is still
moisture in the air. They are also seen often around water falls when
there are water droplets in the air from the splashing water. Remember
that when you are looking for a rainbow after a rainfall, you need to
look away from the Sun. The light needs to shine past you, reflect off
the back of the water droplet and then return to your eyes in order for
you to see the effect.
Similar effects to rainbows are sun dogs
and light pillars
Both of these phenomena occur as light refracts through tiny ice
crystals in the sky, so they occur primarily during the winter months.
Sun dogs, or ‘parhelion’ the scientific name, appear as a ‘halo’ around
the Sun with colours much like a rainbow. This occurs as the light is
refracted through the ice crystals, contained in cirrus or cirrostratus
clouds, down to the observer. The halo appears at roughly 22? around
A Sun Dog is a halo of colourful light around the Sun.
The light is refracted by ice crystals in the winter sky.
Light pillars are a similar phenomenon to sun dogs. They appear as an
extended ‘pillar’ of light usually above or below a strong light
source. The most common instance of light pillars comes from the Sun,
in which case it is sometimes called a Sun pillar, rising on a winter
morning. A light pillar occurs as ice crystals closer to the surface
of the Earth reflect light towards the observer. Essentially, the ice
crystals act almost like lenses, sending additional light towards the
observer than would normally occur. This is slightly different than
what happens in a rainbow or sun dog, as the light is simply reflected
rather than refracted. This means that there is no dispersion of light
and the rainbow colours do not appear.
A light pillar occurs as light is reflected off of ice crystals in the air.
A second effect worthy of mention is noctilucent clouds.
Noctilucent clouds are the clouds at the highest altitude on Earth.
They exist at the level of about 85km above the Earth, within the
mesosphere. They are only visible between the latitudes of 50? and 60?
in both the northern and southern hemispheres, and they are primarily
visible during twilight during the summer months. This gives them their
name, from Latin, meaning roughly ‘night shining’.
clouds appear near the horizon when the Sun is still in fact below the
horizon, leaving the ground near the observer and the lower layers of
the atmosphere still in shadow. Essentially the light from the Sun
shines at just the right angle to bounce off the bottom of these clouds
and reflect down to the observer. They are only visible in this way
because they are so faint that only with the Sun below the horizon are
they bright enough to stand out against the sky.
Noctilucent clouds as seen casting erie colours at sunset here. These special clouds
are only visible when it is still night on the surface of the Earth, but the Sun has already
risen to shine on these high floating clouds.
Noctilucent clouds are not well understood, but they
are thought to be caused by small amounts of water vapour. They are
difficult to study because of their extreme altitude. Weather balloons
cannot fly high enough, so rockets are most commonly used to study this
phenomenon. Recently there has been some evidence that exhaust fumes
from the Space Shuttle may be enhancing these clouds.
is an amazing phenomenon which is difficult to see, but worth the
effort. It is only visible just before dawn or just after dark.
Zodiacal light appears as a faint glow in the sky roughly triangular in
shape, following the path of the Sun, the ecliptic, across the sky.
Although it is faint, on a moonless night it can account for more than
half of the light visible in the night sky. A dark night is needed,
though so a bright moon phase will often wash out a good view of
The triangular glow of Zodiacal light is most often seen in the east, just
before an autumn dawn, or in the west just after a spring sunset. This image
has several of the zodiac constellations (Pieces, Aquarius, Capricorn
and Sagitarius) added to show how the light follows the ecliptic.
The name ‘zodiacal light’ comes from the fact that
the light follows the line of the ecliptic. The constellations along
this line have been named according to the 12 zodiac signs, giving this
light its name.
Zodiacal light is
caused by particles in the ecliptic as well. There is an often
forgotten part of the solar system that is the cause of this
phenomenon. Circling the Sun is a cloud of interplanetary dust,
roughly the shape of lens. It extends out past the orbit of the Earth
and also encircles the Sun. The light of the Sun reflects off this dust
to produce the zodiacal light.
enough, the dust needs only to be sparsely present to create the
effect. It has been estimated that if the dust particles are only about
1mm across, the particles would only need to be scattered about 8km
apart to create the effect we observe.
Did you know?
light has another name: the false dawn. Because this light often
appears before dawn it almost appears as if the Sun is getting close to
rising. Actually it is misleading as the Sun follows some time after.
Zodiacal light will actually fade before the true dawn arrives.