Atmospheric optical phenomena

[ceptimus]

The best-known (and coolest) of these phenomena is the rainbow but there are lots of others - some of them are every bit as common as the but they are not as well known.

Sun dogs: These are most commonly seen at sunrise or sunset and take the form of two (sometimes only one is visible) phantom suns - one on either side of the real sun. The phenomenon is caused by refraction of the sunlight through ice crystals in high cirrus clouds.


photo courtesy of Wikipedia article.

If there is enough cloud, the sun dogs sometimes extend to form a complete arch or ring around the sun - you can see parts of the arch in the above photo.

Many people haven't seen (or even heard of) sun dogs, but they are very common - more common than an 'ordinary' rainbow. If you want to see them for yourself, wait for a day with mostly clear skies and just some high wispy cloud and look out for them around sunset or sunrise. As always, avoid looking directly at the sun itself as this can cause permanent eye damage - use a piece of paper or similar to cover the sun while looking for the sun dogs on either side.

[Petra]

Wow, that's cool! I must say I've never, ever noticed sun dogs before, but I'll be looking out for them from now on.

[Crumb]

So what about a green flash?

[livius drusus]

That is so cool. I only knew sundog as a username. I'll definitely look for them now.

[ceptimus]

Originally Posted by Crumb

So what about a green flash?

These are very rare and difficult to see. I've never seen one They occur for just a second or so at sunset. You need a very clear and stable atmosphere and an uncluttered horizon, so the sun setting over the sea is probably best.

As the sun sinks it turns the familiar red colour, and when there is only a tiny part of it still showing it can turn green, or occasionally blue or violet, before it disappears completely.

There is some dispute over the explanation of the green flash. The primary cause is believed to be atmospheric refraction caused by layers of air at different heights having different temperatures. Atmospheric refraction allows light to be bent so that at times we can see over the horizon - this is the accepted explanation for mirages, which I'll deal with in a later post.

It is known that atmospheric refraction affects shorter wavelengths of light more than longer ones - so when conditions are right for this phenomenon, we can see slightly further over the horizon in the green, blue and violet parts of the spectrum than we can in the red, and yellow parts.

Now, as the sun sets, disappearing over the horizon, we are able to see the green part of the sun's spectrum when the yellow part has already set - and the violet part for even longer. The whole process is over in less than a second, but if you are in the right place at the right time with the right weather conditions, you may be lucky enough to see it.

Remember what I said though about not staring at the sun. It's dangerous. The green flash is only ever going to be seen in the final second as the last sliver of the sun disappears below the horizon, and it's okay to look directly at that as you are only looking at a tiny part of the sun for a second and as it's on the horizon, you'll be looking at it through many miles of protective atmosphere.

Incidentally, the reason sunsets and sunrises are red is due to the fact that you're looking at the sun through a much greater depth of atmosphere than normally, but that is perhaps a topic for a later post.

[Ymir's blood]

Heh, I thought he was making a Pirates of the Caribbean joke.

[JoeP]

Thanks cep!

I think auroras are still the bestest atmospheric optical phenomena. Not that I've ever seen one.

Originally Posted by ceptimus

As always, avoid looking directly at the sun itself as this can cause permanent eye damage - use a piece of paper or similar to cover the sun while looking for the sun dogs on either side.

I was going to castigate you for belittling sungazing, but there's such rich material I'll do it another thread.

[Dingfod]

I always heard that sun dogs meant a storm was coming in about three to five days. A ring (rainbow) around the moon meant much the same. I've seen both.

[Angakuk]

The only time I have seen sundogs was on a winter day in North Dakota, about mid-morning. I happened to be driving north and they, along with arc of light around around sun, were clearly visible for the entire 30 minutes of the trip. One of the local ministers later reported that he had seen them as well. Being completely unfamiliar with the phenomenon he tried calling his neighbors to ask what they were. Call after call went unanswered. He could even hear the phones ringing in his neighbors' houses, but no one was home. He was beginning to seriously consider the possibility that this was a sign of the end times and that he had missed the rapture. Finally someone answered their phone and said "Oh, that. Those are just sundogs".

I saw the Aurora Borealis once in Alaska, on Halloween night. It was accompanied by a weird whistling sound. I later learned that the whistling was caused by the children of the village, there being a local tradition that if enough people whistle loudly enough it will make the lights dance faster.

[ceptimus]

The Brocken Spectre: This is one that I have been lucky enough to see: once when hill walking in the mountains of Snowdonia, and several times while flying in aircraft above clouds. Of course, on each of these occasions, I didn't have a camera with me.


This photo courtesy of Richard Cross' website

The phenomenon is named after The Brocken, a hill in Germany where it was often observed and reported. It takes the form of an apparently enormously magnified shadow, on cloud or mist, surrounded by a rainbow-coloured halo.

To see the Brocken Spectre best you need to be above cloud or mist, with a low sun behind you casting your shadow onto the mist. The apparent huge size of the shadow is an optical illusion due to your perception that the shadow is much further away than it really is. The halo or 'glory' of colours surrounding the shadow is caused by the sunlight reflecting back from water droplets in the mist - for the glory effect to be pronounced, the water droplets must all be the same size - this is quite rare, and explains why the effect isn't often observed.

When several people are walking together, they will all see everyone's shadow, but each person will only see the coloured halo surrounding their own shadow.

[S.Vashti]

The Photographic Weather Reference Library. I have this bookmarked, because one day I was looking at some weird clouds, and wanted to know if they are a certain type. They were--virga, which is a cloud formation artifact.

[ceptimus]

Mirages: All of us will have seen one form of mirage - on a hot day when the road appears to be covered in shimmering water, when no water is really there.


Image courtesy of Wikipedia

The more impressive variety of mirage commonly appears over deserts or bodies of water. Images of objects that are many miles over the horizon, often distorted, appear to float on the horizon or sometimes above it. In polar seas the mirages are often of icebergs, but they can sometimes be distant land masses or even cities.


More info on this image here

In all cases, the mirage is caused by refraction of light by layers of air at different heights having different temperatures and/or moisture content.

[Watser?]

Saw an old-fashioned rainbow today when I was going to my sisters house for a family bbq and when I got there there was a garden gnome that they swore wasn't there before

[Dingfod]

Gigantic jets are a form of lightning connecting the tops of thunderstorms to the ionosphere. They shoot up as high as 48-50 miles and were only recently documented, most recently in Broken Arrow, Oklahoma, by an amateur astronomer using a low light video camera trying to capture images of a meteor shower. Other electrical discharge atmospheric events documented are blue jets, sprites and elves. Yes, that's what they are called.

APOD (Astronomy Picture of the Day) video link

[ceptimus]

Halo Cloud effect: I snapped this (admittedly poor quality) photo today on my mobile phone. When a cumulus cloud is lit from behind by the sun (or sometimes the moon) the dense middle part of the cloud blocks the light and so appears dark, but when we look at the edges, we are only looking at a thin layer of mist - we see the edges brightly lit, which can give a halo or 'silver lining' impression.

[Dingfod]

That there is what we called a 'Jesus cloud' when I was a kid.

[Plant Woman]

I've seen the green flash 3 times when I lived in Hawaii. We lived in a beach house on Sunset beach where almost every night when there was a clear horizon we had a green flash party. Neighbors would gather on our lanai and we would unwind from the day and watch for the green flash. It took a long time before I saw one, and when I did it was a split second of green and then it was gone.

[ceptimus]

Nacreous clouds are very thin filmy clouds that occur very high in the atmosphere.

When seen in an otherwise clear sky, before sunrise or after sunset, they have beautiful iridescent colours. The reason they shine, of course, is that although it is dark on the ground, the clouds are high enough so that the sun can illuminate them.

They are quite rare phenomena, and mainly seen near the arctic or antarctic.

[naturalist.atheist]

Does it have to be earth's atmosphere? There is the giant red spot on Jupiter, and then there are lunar flashes.

SkyandTelescope.com - Planets - Transit Times of Jupiter's Great Red Spot

SkyandTelescope.com - News from Sky & Telescope - Lunar Flash Doesn't Pan Out

Return to the Moon - Lunar Science

[Radathser]

Explain this.

A few weeks [or months] ago I was outside with a friend and we witnessed something no one we knows seems to have any clue of. We both saw it and have recalled it several times since, so it was not a dream. We were outside with the sun still a good distance from the horizon at about 7-7:30pm and all of the sudden, the only term I can come up with is, "Red shift."

The sky turned red and red light flooded everything outside. The grass appeared an awesome color. I remember best the sky, red, like Mars. If you looked straight up, it was like landing on a future terraformed Mars. It was still very light outside and was just getting into evening. There was, -no- blue in the sky, and I repeat, no blue. There were not varying shades of orange and yellow typical of evenings. The sky was red, communist red.

Explain that, as I'd really like to know what the hell that was. It lasted for several minutes if not longer. I would theorize that the introduction of an uncommon element into the sun might change the light it emits for a short time, but then we'd see something about it in the news, and we never did. It's probably much more simple than that, but I'm just missing it. I figure it's probably some variation of a rainbow, or something to do with our atmosphere, but the amount of red light that saturated the sky and the suburban environment around us was just awesome to behold. I wish it had lasted longer. I really got a feeling for what the Martian sky might look like.

[Dingfod]

LSD will do that.

[Radathser]

Originally Posted by Dingfod

LSD will do that.

Decent hypothesis. However, I don't do drugs, and neither do any of my friends, or that friend in particular.

[Dingfod]

I can't think of any reason for the sky to turn red. Maybe one of our scientist-types can come up with an explanation.

[Radathser]

I hope so. I don't want to have to commit myself and my friend to an insane asylum [pft, they'll never catch me!].

[Doctor X]

We will catch you . . . when we want to.

--J.D.