Thanks to the Ridge Wallet for sponsoring today’s video. Have you
ever looked up at the sky and seen something so strange it made you wonder if you were dreaming?
Perhaps you saw what looked like a rainbow; only it was upside down! Or maybe you saw a perfect
circle of light around the sun with two splashes of rainbow colour on the sides. Well, no need to
pinch yourself: what you were seeing is a unique category of optical phenomena called ice halos.
Although similar, these aren’t technically rainbows but their own distinct wonder.
Some are relatively common, while others are extremely rare. As you might know, rainbows
occur when sunlight is refracted by water droplets suspended in the atmosphere. When light travels
from one transparent substance to another, such as from air to water, its speed changes, bending its
path and separating the light into coloured bands by wavelength – a process called dispersion. But
what happens when light hits water particles that are made of ice rather than liquid water?
Well, depending on the shape and orientation of the ice crystals, as well as the direction of
the light hitting them, they can produce a whole range of optical phenomena. To date, 119 kinds of
ice halos have been categorized, and there might still be others we don’t know about. So, what
are ice halos? How do they form, and what should you look for if you want to see one for yourself?
And finally, are they a sign of something more ominous on the horizon? I’m Alex McColgan, and
you are watching Astrum. Join me today as we look at spectacular images of some of the better-known
ice halos and get to the bottom of how – and why – these delicately beautiful spectacles appear.
This stunning photograph shows a 22-degree solar halo spotted over Grosskrotzenburg, Germany.
“22-degrees” doesn’t refer to air temperature, but to the angular radius of the halo.
To visualize this, imagine drawing a direct line from a point on the halo to the observer’s eye.
Then do the same from the sun at the circle’s centre to the observer’s eye.
The angle between these two sightlines is 22-degrees, hence the name 22-degree halo.
There are also 46-degree halos, which, as you might guess, are larger than their 22-degree
cousins but also fainter. Halos can also occur in moonlight, as you see here. These eerily beautiful
halos are called moon rings or winter halos, but they’re technically the same optical phenomenon.
What distinguishes them from solar halos is the source of light. In some images you may notice
that the patch of sky inside a halo appears darker than the sky outside the ring, a dramatic effect
that is sometimes described as a “hole in the sky.” This happens because, when light bounces
and refracts off the ice crystals, none of it is reflected toward the light source, which, to
an observer, leads to an apparent darkening inside the halo. But this is just the tip of the iceberg,
because ice halos take on all kinds of strange and shocking forms: spots of vibrant colour,
“upside down rainbows” and even “upside down rainbows” at night! But before we get there,
let’s take a moment to break down the fascinating optics that produce these strange occurrences.
Most circular halos are made by water crystals in cirrus or cirrostratus clouds. These 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
very hot climates! Atmospheric ice crystals are small (usually less than 10 micrometres long) and
uniform in size and shape. This extreme regularity has to do with the structure of ice on a molecular
level and is necessary for making halos. You see, water molecules consist of two hydrogen atoms
and an oxygen atom. In liquid water, the bonds between hydrogen atoms are weak, so the bonds are
constantly breaking and reforming as the molecules slip past each other. But when water freezes,
that molecular movement slows to a halt and the bonds between hydrogen atoms become fixed. This
results in a hexagonal lattice structure known as a tetrahedral arrangement. This highly organized
structure on the molecular scale is what causes the ice particles to form hexagonal crystals. Some
crystals are flat, like plates, whereas others look like columns with a hexagonal cross-section.
Despite this variation, these crystals behave so uniformly because of their interfacial angles.
In fact, the angles between any two faces of ice crystals are completely identical.
It’s this extreme regularity of the interfacial angles, rather than the crystals’ mass or shape,
that allows millions of particles to scatter the light with the constancy needed to produce a halo.
Halos are sometimes quite colourful. That’s because, when ice crystals refract light,
the red and blue frequencies bend at slightly different angles and disperse.
Some 22-degree halos have two colourful spots visible to the left and right of the Sun.
These beautiful-looking phenomena are called parhelia, or sun dogs. Sun dogs appear when
ice crystals that are shaped like hexagonal plates take on a horizontal orientation.
As I mentioned earlier, some crystals form as thin hexagonal plates whereas others form as columns
with hexagonal cross sections. When plate-shaped crystals form and settle downward, atmospheric
drag orients them horizontally, with the C-axis you see pictured almost perfectly perpendicular.
Larger plates tend to wobble just a bit more than smaller ones. This wobble, in turn,
affects how tall sun dogs appear to be, with wobblier plates causing the sun dogs to appear
more vertically stretched. Sun dogs are sometimes accompanied by another optical phenomenon
called a circumzenithal arc. These arcs look similar to rainbows, only they’re upside down,
which gives them a friendly, smiling appearance. These arcs appear over the
sun only when the direction of the light and the orientation of the ice crystals are just right,
when light enters the top faces of hexagonal plate crystals and exits the side faces. However,
this can only happen when the Sun is lower than 32.2 degrees above the horizon and more than 5.
Because the ice crystals function essentially like a 90-degree prism, the colour separation
is excellent, meaning circumzenithal arcs have a far higher degree of clarity than actual rainbows.
And if you’re really lucky, you might even spot one with moon rings.
Called lunar circumzenithal arcs, these unusual arcs only appear when the moon is extremely bright
and fairly low on the horizon. As strange as it sounds, it is possible to see what
looks like an upside-down rainbow at night! Now, this is amazing: circular halos have
also been sighted on Mars! Here is an image taken by Perseverance on December 15, 2021.
This has led some to suggest that the cause may be carbon dioxide crystals in the planet’s cold,
thin atmosphere, or perhaps a mixture of CO2 and water ice. We didn’t always know that
halos occurred on Mars, so as you can imagine, the Perseverance team was pretty excited to see this.
Because the size and appearance of the halos provide clues about the types of crystals
forming in the atmosphere, scientists hope that studying halos here on Earth and spotting them
on other planets could shed light on the composition of those planets’ atmospheres.
As a result, more research will be undertaken in the future to document
and understand the appearance of ice halos on Mars, as well as other moons and planets.
So, there you have it, a primer on some of the better-known ice halos and their associated
optical phenomena. And if you want to see one for yourself, here are some things to look for.
First, check for cirrus clouds. These are the high-altitude, wispy clouds that contain
the small ice particles needed to produce ice halos. Second, look for the Sun or Moon (but
take care not to stare directly at the Sun, kids. Because that can damage your eyes!)
While rainbows form around the antisolar point, which is located directly opposite the Sun in the
celestial dome, most ice halos form in proximity to the Sun or Moon. And while ice halos tend to
appear in fair weather, they often arrive at the front of an approaching storm system. Meaning,
if you see one, it’s better not to linger outside for too long. Head indoors and find somewhere cosy
to enjoy the upcoming storm. So, if the weather is fair but you see rain in the 24-hour forecast,
keep an eye on the sky: your chances of seeing a halo will be higher. Although, of course,
the best way to see one is to look every day. Ice halos can form anywhere on Earth,
so if you keep looking up, you’re bound to see one sooner or later! But when it comes to ice halos,
there’s plenty more to talk about. There are lots of other strange and even rarer kinds of halos,
and probably more yet to be discovered. So, if this is a topic you’d like to see more about in
the future, and if you’ve seen them for yourself, I’d love to hear about it in the comments.
If you’re heading out the door to go and try see an ice halo, it’s a good idea to never leave home
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Thanks for watching. If you like atmospheric phenomena, check out this playlist for more.
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name added to this list too, check the links below. All the best and see you next time.