Imagine seeing a glowing light suspended in the air.
It looks almost like a duplicate Sun, blindingly bright and fixed at some point low on the horizon.
Suddenly, the clouds pass overhead, and the dazzling light vanishes like vapour.
No, this isn’t a scene from a fantasy movie, but an optical occurrence called a subsun,
which is one of several uncommon kinds of ice halos. If you watched my previous
video on ice halos, then you already know a few of the better-known types. To recap,
halos are optical phenomena that appear when light hits tiny ice crystals that are suspended in the
atmosphere. Depending on the shape and orientation of the ice crystals, as well as the direction of
the light hitting them, they can take on a wide variety of appearances. Scientists have identified
119 kinds so far, and there may be others we don’t know about. Today we’ll look at a few of
the rarer and more unusual kinds of ice halos. As you’ll see, these phenomena can be very strange,
and the best way to understand them is as optical illusions that don’t truly “exist” in space.
Nonetheless, if you happen to be in the right place at the right time, you’ll be treated to
some dazzling sights that test the boundaries of what is real and what is an illusion.
I’m Alex McColgan, and you are watching Astrum. Join me today as we look at
stunning high-definition images of some of the rarest and most unusual ice halos,
including subsuns, Bottlinger’s rings, light pillars and more.
Let’s start with a little primer. Ice halos are produced by water crystals that are usually found
in cirrus or cirrostratus clouds. Cirriform clouds occur high in the troposphere, usually
5 to 10 kilometres above the Earth’s surface, where temperatures are very cold. As a result,
halos can be seen anywhere on Earth all year long, even in hot climates. These ice crystals are very
small (usually less than 10 micrometres long) and always have uniform interfacial angles – so
uniform, in fact, that the angles between any two faces in an ice crystal are completely identical.
However, despite this angular regularity, ice crystals vary in size and shape. Some crystals
are flat, like hexagonal plates, whereas others form as columns with a hexagonal cross-section.The
subsun we looked at earlier requires plate-shaped crystals to occur. As the plate crystals settle
downward, atmospheric drag orients them parallel to the Earth’s surface (much like how sundogs
occur). Subsuns form when disturbances in the air introduce a wobble to these plate-shaped
crystals. The more wobble the plates have, the more vertically elongated the subsun will appear.
Occasionally, there is so much vertical elongation that the
plates produce a categorically distinct type of halo known as a sun pillar.
These only appear when the Sun is between 20 degrees and 6 degrees above the horizon. However,
while sun pillars might look like beams of light shooting from the ground, this effect
is deceiving.. What’s really happening is that the plate crystals act like millions of tiny mirrors,
which bounce the light rays toward the observer along a vertical axis. These scattered rays then
produce a diffuse reflection visible to an observer as a pillar of light.
A similar effect happens when the Sun sets over water, bouncing a diffuse reflection off the water
surface that sometimes looks like an elongated pillar. What do you think: see a resemblance?
Sometimes, light pillars can appear when the Moon is the primary light source.
As you might have guessed, these are known as moon pillars. This image shows a beautiful one
captured by the European Space Agency over Antarctica. However, when the conditions are
right, light pillars can come from all kinds of sources, even artificial ones. On occasion, this
has led to some dramatic scenes. In this photo, low-forming ice crystals called diamond dust have
settled close to the Earth’s surface, creating a spectacle that looks almost like an alien arrival.
And if you haven’t heard about the peculiar effects of tiny ice crystals suspended in the
Earth’s atmosphere, you can hardly be blamed for jumping to conclusions. I wonder how many
eyewitness accounts of mysterious lights can be explained by halos like this one.
Let’s look at a different but equally strange phenomenon called Bottlinger’s rings.
First documented during a hot air balloon flight in 1909, Bottlinger’s rings usually last for only
a few seconds, although on occasion they can be visible for upwards of an hour.
They typically appear as shimmering elliptical rings around the Sun, but due to the Sun’s intense
glare, they’re more likely to be sighted around dimmer subsuns. Most sightings of
them tend to occur in airplanes. Bottlinger’s rings were originally thought to be produced
by plate crystals that had developed a gyrational motion. However, simulations have recently shown
that the effect can be more effectively reproduced by pyramidal-shaped crystals
with a small amount of wobble. Still, there’s a lot we don’t know about Bottlinger’s rings,
and the current theories are mostly educated guesswork. The pyramidal crystals that work in
simulations have very wide apex angles, such that they take on a flat, almost platelike appearance.
The problem is these shapes are unlikely to form naturally from ice’s molecular lattice structure.
All we really know is that these elusive halos are extremely
rare. Which theory do you find more convincing? I’d love to hear it in the comments. Lastly,
I’ll leave you with a rare cousin of the circular halo called supralateral arcs.
Unlike 22-degree halos, supralateral arcs never form a complete circle,
although they sometimes form at the same time as circular halos, producing a complex halo effect.
Supralateral arcs appear as coloured bands above the Sun but never reaching below it. When their
colours are visible, red is the band closest the Sun with blue on the outer edge. We think
supralateral arcs form because of hexagonal rod or column-shaped crystals, when light enters the base
and refracts through the prism side. While these arcs are sometimes confused with 46-degree halos,
which are a larger cousin of the 22-degree circular halo, they are a separate phenomenon
that visually resemble rainbows due to their arc shape and sometimes colourful appearance.
So, there we have it: a journey through the dazzling and illusory world of rare and
unusual ice halos. And as I mentioned earlier, halos are still a growing area of research,
and there may be other kinds of even rarer halos that have yet to be documented. So,
if you’ve ever seen anything strange and hard to explain, I’d love to hear it. It’s possible you’ve
seen something that scientists don’t know about yet! Thanks for watching and see you next time.