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.
