An Alphabet of Atmospheric Optical Phenomena

by | 4 Feb, 2025 | Astronomy, Nature, Weather | 0 comments

It’s no secret that I like the sky, particularly clouds and atmospheric phenomena (which are the phenomena caused by light as it interacts with elements in the air or atmosphere. A rainbow is a well known example.)

As a result, I have many photographs of the sky hidden away on hard drives which could, at this rate, potentially never again see the light of day.

And this seems a bit of a shame.

I don’t know if it’ll be of interest to anyone else, but it seems remiss to keep all my enthusiasm, and photographs, to myself, so I’m toying with the idea of writing about the different atmospheric phenomena and clouds I’ve seen and, where possible, sharing my photographs. (I can’t promise I’ll have photographs to illustrate every phenomenon I’ve observed, but I will share those that I do.) 

I’ve found an alphabetic list of atmospheric optical phenomena on Wikipedia. It seems like a sensible place to start, as I can tick off the phenomena I’ve seen while keeping an eye out for those I have not. (I’ll deal with clouds in a separate blog.)

Here are the phenomena from the list beginning with A:

  • Afterglow
  • Airglow
  • Alexander’s Band
  • Alpenglow
  • Anthelion
  • Anti-crepuscular Rays
  • Aurora

That’s quite a lot to cover in one blog post, but let’s see how I do.

Afterglow

Afterglow is defined as an intense red glow in the atmosphere seen long after sunset when most of the twilight colours should have disappeared. It’s caused by dust in the stratosphere and reading about it has answered a question I’ve had for a while. On still, clear evenings, I like being outside at dusk because I enjoy watching the procession of colours as daylight fades. But I’ve often wondered why it is that sometimes the horizon turns vermillion when other times it does not, even though the weather conditions seem largely the same. Dust in the atmosphere would explain it; the more dust there is, the more colour, as the dust catches the hues of the twilight arch. 

Here is a photograph I took in which I think I’ve captured afterglow. 

Afterglow. Venus sets above The Cobb in Lyme Regis, Dorset. Photograph by Kerrie Ann Gardner

It was taken many moons ago in December, between Lyme Regis and Charmouth in Dorset, UK. This section of beach is cut off at high tide, but during low tide it is possible to walk between the two towns on a thin, fossil-peppered strip of sand and clay. I’d gone there specifically to see Venus hanging just above The Cobb, the famous harbour wall in Lyme Regis, but I hadn’t expected such intense colours. To this day, it still remains the most intense afterglow I’ve ever seen.

Airglow

Ah, airglow. This phenomenon had me very confused. When I first saw it, I was in Cornwall on a very badly thought out photography mission. I’d been to the Eden Project to see an evening concert and then decided it would be a sensible action to drive down to the former coastal tin mine of Wheal Coates, somewhere I had never been before and, as you’d expect, a location that was very tricky to find in the dark, especially without such useful tools as Google Maps. I managed to find it some time between midnight and 1am, and set about trying to photograph the old mine with the backdrop of the Milky Way.*

*At this point I should probably let you know that I am an enthusiastic yet largely unskilled astrophotographer. I often try my luck but the quality of the resulting photographs is debatable because I rarely take the time to stack them as more thorough astrophotographers do.

Back to airglow. Something was wrong. Every photograph I took seemed to have light pollution in it; a weird, all-pervading greenish glow which looked to be everywhere above me. It was unlike any light pollution I’d seen before and I remember being crestfallen that I’d travelled all this way only to have my photographs ruined by artificial light.

Airglow at Wheal Coates, Cornwall. Photograph by Kerrie Ann Gardner

It was years later that I realised that what I’d seen that night was in fact airglow, an intriguing phenomenon caused by atoms and molecules in the upper atmosphere which, having been excited by sunlight during the day, emit light at night as they shed excess energy. (This is known as chemiluminescence.) Although it was officially identified in 1868 by Anders Jonas Ångström, airglow was known to the ancient Greeks, who called it Chasmata. In very dark locations it can be seen with the naked eye, but it’s easier to pick out with a camera. Airglow and the aurora both occur in the upper atmosphere, which is why they are similar colours. But whereas the aurora is fleeting, caused by the interaction of solar energy and the Earth’s magnetic field, airglow is constant, and envelopes the entire globe with a subdued bubble of light about a tenth as bright as all the stars in the sky.

To find out more about airglow, please see here, here and this video here.

Alexander’s Band

If you look closely at a double rainbow, you might notice a darker area between the primary and secondary bow. This is Alexander’s Band (also known as Alexander’s Dark Band) after Alexander of Aphrodisias who first described the effect in 200 AD. Like the rainbow, this optical phenomenon is caused by refraction. Sunlight enters a raindrop, where it is split into the colours of the spectrum before bouncing back to the observer’s eyes. When two rainbows are visible, the light has bounced inside the raindrops twice. (Some of the light is lost during this second bounce, which is why the secondary rainbow is always fainter than the first.) Alexander’s Band is caused because the light between the two bows is being scattered at angles which don’t reach the observer’s eyes.

You can see Alexander’s Band (plus a number of raindrops on the camera lens) in the photograph below.

Alexander's Band Double Rainbow photograph by Kerrie Ann Gardner

And here’s a close up, with some Rooks…

Alexander's Band double rainbow photograph by Kerrie Ann Gardner

Alexander’s Band is not restricted to rainbows. It can also be seen in glory’s, too, the halos that seem to surround an observer’s shadow in dense mist of fog. But we’ll get to those in another blog post.

Alpenglow

Alpenglow, from the German ‘Alpenglühen’ (meaning Alps Glow), is a reddish glow which can be seen on mountaintops around dawn and dusk. Whether it is caused by direct or indirect sunlight, however, is another matter. If it is direct sunlight the explanation is simple – the rising or setting sun is close to the horizon, so its light, having passed through a greater area of atmosphere, appears red (as the shorter, blue wavelengths are scattered by the atmosphere, allowing only the longer, red wavelengths through). This light falls on mountain peaks while the rest of the landscape is enveloped in shadow, causing them to turn a rosy red, as in the photograph below.

Alpenglow as seen on the mountains close to Knoydart, Scotland. Photograph by Kerrie Ann Gardner

If the effect is caused by indirect sunlight, however, the light is reaching the summits another way. It is being reflected from particles in the atmosphere (dust, ice crystals etc) which cause the backscattering of red light. This is the same process which creates the Belt of Venus, a pinkish band which sits above Earth’s shadow in the sky opposite the recently set or soon to rise sun. In theory, mountain peaks lit by indirect sunlight would have less pronounced shadows than those lit by direct sunlight. From what I can glean from the internet, this is the more accepted reason for Alpenglow, although it seems this phenomenon causes disagreements because it is all relative to the observer. Being lower down, an observer at sea level might see peaks which seem to be lit indirectly because from their position the sun has set. An observer half way up a mountain, however, will see those same peaks lit directly, as their higher elevation will enable them to still see the sun.

Alpenglow on the distant peaks of the Isle of Rum, Scotland. Photograph by Kerrie Ann Gardner.

Photographer Dan Baily has written a convincing argument stating that Alpenglow always results from direct sunlight – and being that he flies above mountains at twilight, giving him a perspective few of us have, I think he might have the answer. You can read his blog here. In the end, it seems it’s all a matter of perspective. I was at sea level when I took the above photograph of the Isle of Rum. From my standpoint, that rosy hue was being caused by a sun which was yet to rise. But was the sun really still below the horizon? As I can’t be at two places at once, I will never know.

Anthelion

This is one of the halo phenomenon which I am yet to see and I would like to know more about. According to an article on Wikipedia, it is a member of the halo family which appears opposite the sun, and it looks like a faint white spot crossed by an X-shaped pair of diffuse arcs. I’ve searched through Google Images for an anthelion, and here is one of the best images I’ve found. I’ll keep looking for this one.

Anti-crepuscular Rays

Currently, and somewhat shamefully, I do not yet have a photograph of this atmospheric optic. The reason, I think, is that I have been too preoccupied with its counterpart – Crepuscular Rays, which appear as sunbeams radiating from behind a cloud. Because they’re so captivating, all my attention has been given to these vast beams of light, which means I have, regretfully, forgotten to turn around and look at the sky behind me. And it is here, if I had turned around, that I might have spotted Anti-crepuscular rays, for those same beams and shadows caused by clouds and tiny particles in the atmosphere may well have reached right over my head and all the way to the opposite horizon, where they would have converged at a vanishing point directly opposite the Sun, making them anti-crepuscular rays. But alas, I have hitherto not turned round, so I have not seen them.

*Edit* A few evenings after writing this I went looking for the Green Flash, another atmospheric optic I am yet to see. (I didn’t see it.) I did, however, turn around after sunset, and happily there in the sky, albeit very faintly, were some anti-crepuscular rays. I took a photograph, but you’re going to have to look very closely to see the rays. I promise they’re there though. Turn your brightness up and look at the centre of the horizon. You should see some faint pink rays radiating diagonally towards the left and right of the picture. I admit it’s not easy. For a better photograph of this phenomenon, see Will Gater’s, here.

Anti-crepuscular rays photograph by Kerrie Ann Gardner

Aurora

We are now in solar maximum, which is the period of time when the sun is at its most active. Happily, this means that we are more likely to see the aurora. But there are some downsides, too – not all geomagnetic storms are benign. Some, like the Carrington Event of 1859, which is the strongest geomagnetic storm on record, caused worldwide telegraph disruption and aurorae so bright that gold miners in the United States thought it was morning, while in Australia gold diggers in Perth saw: ‘Lights of every imaginable colour […] issuing from the southern heavens, one colour fading away only to give place to another if possible more beautiful than the last.’ A similar event today would likely be, to put it mildly, catastrophic, as it would not only disrupt radio signals and satellites (bye bye GPS) but also wipe out our electrical grid, which we rely upon so much these days. More alarmingly, there is mounting evidence that even stronger geomagnetic storms have hit the Earth in the past. If a storm of such magnitude bombarded the Earth again it would be extremely bad news.

Now that I’ve frightened you with the dark side of geomagnetic storms, let me move onto their brighter side. As someone who’s spent a long time trying and failing to see the aurora during solar minimum, having an increased chance of seeing the lights has been fantastic. During the last few years I’ve been fortunate enough to see some incredible displays from my 50º north vantage point. I even heard the lights back in April, 2023. (Yes, you read that correctly. If the conditions are right, it is possible to hear the aurora.)

Aurora, 12th August 2024. Photograph by Kerrie Ann Gardner.

Aurorae are produced when plasma from the sun interacts with the Earth’s magnetic field. This is always happening to an extent (hence airglow) but it’s when the Sun blasts out more particles than usual that we have a better chance of seeing the aurora. This can happen when a coronal mass ejection (CME) – a large expulsion of plasma from the Sun’s corona – washes over the Earth. If the interplanetary magnetic field is favourable and it’s a direct hit, the likelihood is we’ll see aurora. And as for the colours, they’re produced by solar particles interacting with gases in the Earth’s atmosphere. Excited by the particles, these gases exhibit different colours: lower altitude oxygen tends to give off green light, nitrogen glows blue and purple and during bigger solar storms, the oxygen floating 150 miles above the Earth glows red.

The image below was taken during the Great Aurora Display of the 10th-11th May, 2024, from Dartmoor, UK. It was an exceptional night which left me utterly spellbound. The aurora was so intense it was easily visible with the naked eye, and viewers were treated to a kaleidoscope of colours as towering beams of light slowly billowed through the sky. I am so grateful that I had the good fortune to see this display. If you want to read about it in more detail my blog post can be found here. There is also a lovely video of this display by Paul Haworth on YouTube. It’s worth a watch if you have a spare half an hour.

Aurora from Dartmoor, 10th - 11th May, 2024. Photograph by Kerrie Ann Gardner.

The aurora has many shapes. It can be seen as diffuse, a glow or an arc, as beams, blobs, pickets, curtains, rays, or a corona, and also as a sub auroral arc (SAR) and a Strong Thermal Emission Velocity Enhancement (STEVE). That said, SAR and STEVE are not entirely the same as the aurora. Although they happen during geomagnetic storms, these phenomena are seen at lower latitudes. STEVE is an especially feisty phenomenon which moves incredibly quickly and is around 3,000ºc. It’s thought to be a result of extreme thermal and kinetic energy in the Earth’s atmosphere and it is a mind-bending thing to see. I was overjoyed to see it during an intense aurora display which happened on the 10th-11th October, 2024. STEVE appeared right above our house, and seemed to shoot from the eastern sky and rapidly reach westwards like an intense ribbon of light. It felt like it only lasted a handful of minutes, but when I look back at my photographs from that night I see that it actually stayed in the sky for around twenty minutes before it disappeared.

STEVE aurora seen from Devon UK, on the 10th-11th October 2024. Photograph by Kerrie Ann Gardner.

Contrary to popular belief, aurora is not always confined to the poles. Although activity there is more frequent, big solar storms push the aurora to lower latitudes. In 2024, sightings were reported from India, China and Florida. So if you think you live too far south to see the Aurora Borealis, or too far north for the Aurora Australis, take heart. If solar activity is elevated enough, the Mirrie Dancers (as they’re called in Shetland, ‘mirr’ meaning ‘to shimmer’) can frolic all the way down towards the equator. For your best chance of seeing them, head somewhere free of the light pollution with a good view of the northern horizon. Wait for your eyes to become fully dark adjusted (which takes around twenty minutes) and be patient. A good display is easily visible with the naked eye, although the bright colours are not always evident, so the beams of light may sometimes look whitish instead of red or green. To get the full measure of the colours filling the sky, use a camera, and leave the shutter open for around ten seconds. With enough geomagnetic activity, aurora can appear all over the sky, even when you look south.

***

Phew! That was quite a long blog post. Well done for making it this far, and thank you for taking the time to read it. In my next blog, I’ll focus on atmospheric phenomena that begin with the letter B. I’ll also keep looking for that pesky anthelion.

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