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Circumhorizontal arc

PostPosted: Sun Jun 14, 2009 5:10 pm
by chip
I have just seen what I believe to be a Circumhorizontal arc
high in the sky today at 16:00. It looked like a smaller blurry rainbow above the sun. I understand that these are rare phenomena.
I wonder, how might one take a good photograph of these on such bright days? What equipment and techniques would be best?
Has anyone taken a photo on this forum? Please lets see.

Thanks

Chip

RE: Circumhorizontal arc

PostPosted: Sun Jun 14, 2009 8:13 pm
by brianb
Was it curved? With the arch pointing upwards as in a normal rainbow? If so what you saw was an incomplete solar halo - they come in two varieties, the one 22 degrees from the sun is by no means uncommon though many people don't see it through simply not looking up, especially towards the sun!

The circumzenithal zrc is rarer but by no means extremely so. It has the arch pointing downwards i.e. the ends are curving away from the sun.

Both these phenomena (and many more) are caused by the action of sunlight on ice crystals - when these are regular and nicely aligned they act as tiny prisms.

Photography is easy, just use an ordinary digital camera, holding this in a position where it shields your eyes from direct sunlight. Use manual exposure setting as the meter will be confused by the sun. You want to underexpose rather than overexpose in order not to burn out the image, and the setting you want will be unusually high - something like 1/1000 sec at f/16, ISO 100.

Remember when observing these things that the sun is dangerous to eyesight, even if wearing sunglasses. Try to find a position where the sun is hidden by a tree, or use your arm as a shade for your eyes. Strong but neutral shaded sunglasses are useful for observing these phenomena.


RE: Circumhorizontal arc

PostPosted: Sun Jun 14, 2009 8:46 pm
by bajr
Whilst i was out and about last weekend i witnessed a solar halo. Luck would have it that i did'nt have a camera with me. For a lot of years i've been trying to photograph halos,arcs and sundogs. Last weekend was only the second time i've seen any of these phenomena, so i'm thinking they must be pretty rare. Still to this day i have never seen a sundog.

Brian

RE: Circumhorizontal arc

PostPosted: Sun Jun 14, 2009 10:10 pm
by brianb
[quote]so i'm thinking they must be pretty rare. Still to this day i have never seen a sundog. [/quote]
Haloes - even if fragmentary - can be seen about 1 day in 3. Sundogs maybe 1 day in 5. Other phenomena are rarer. Textbook examples are fairly rare but you will see them if you keep a lookout ... when the sky is milky, so long as there is high thin ice cloud (cirrus) you stand a good chance.

Here's a textbook example of a pure 22 degree solar halo:
Image

and one of a lunar halo - exactly the same except at night:
Image

This is a parhelic arc. The bright object on the right hand side is the "left" sundog which is often seen as a brightening of the 22 deg halo, but not in this case) and the "tail" is parallel to the horizon .... it can extend right round the horizon to the anti-solar point, meeting up with the parhelic arc from the opposite side, though this is very unusual in the UK.
Image

I do not have a really good image of a circumzenithal arc, these are rarer & less bright. But I will get one.



RE: Circumhorizontal arc

PostPosted: Sun Jun 14, 2009 10:26 pm
by bajr
Thank you. I'll keep looking. The lunar halo i've seen many times - but the solar phenomena .....Phhh it's just not happening for me .

Brian

RE: Circumhorizontal arc

PostPosted: Mon Jun 15, 2009 9:33 am
by psychobilly
A Very nice selection of images here...

http://www.atoptics.co.uk/


RE: Circumhorizontal arc

PostPosted: Tue Jun 16, 2009 9:56 pm
by lukeskywatcher
I saw a sun halo a few weeks ago. It was like a rainbow going all the way around the sun. I could only see the full spectrum of colours where the light from the sun passed through (reflected off of) some scattered cloud. It really was a sight to see.