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I'm speaking at a .. by ahendroff   (Show all posts)
Interesting accoun.. by khkk   (Show all posts)
Mountain Rescue Te.. by simon3   (Show all posts)
Thought I'd post s.. by paulca   (Show all posts)
found the explanat.. by jackill   (Show all posts)
Can give no assist.. by Conor74   (Show all posts)
simon3
2011-08-16 13:17:29
"Laser Beam" from simon3 Expand pics
Laser Beam (Expand pics)
Lasers in the mist
On the top of Stumpa Dúloigh jackill did his usual thing of assessing where the highest point actually is. MountainViewers have tended to use the Abney Level as a way of assessing this. In this case the highest point appears to be some metres from the junction of two fences. jackill was assessing the distance from this junction to the highest point where I was with the camera. He has a handy little laser rangefinder gadget for this which showed the fence junction to be 21m from the highest point.

The laser light shows up nicely, being scattered by the misty air. Shows up that is except when it was getting near to the camera - something that begs explanation. As anyone knows who takes photos in the Irish mountains, mistiness is a huge issue. In this case the mistiness is Mie scattering caused by water droplets. These are rather large compared to the scattering caused by small particles that causes blue skies (Rayleigh scattering). Mie scattering also has another important quality. Most of the scattered light goes forward close to the original line of travel. So when viewing the droplets in the beam from a distance you see them because you and they are nearly in line. However, nearer, the angle is different and you can't see the beam.

It's all beautifully explained here:
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html

And the relevance? Well knowing about the two types of mist and what, if anything, you can do about them is really useful for upland photographers. For example, because of forward Mie scatter the sun or the moon tend to get a whitish halo around them when there are large particles in the air. The same doesn't happen for Rayleigh scattering - you don't see a blue halo around the sun. This laser beam is a great example of the physics.
Can anyone explain.. by Dessie1   (Show all posts)
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RECENT CONTRIBUTIONS 1 2 3 .. 17 Next page >>
Track
Central Dingle Quartet from Anascaul
Bunsen7 13 hours ago.
Follow the track from Anascaul lake car park northwards up onto the mountain plateau.Track leads northwards from lake... walk, Len: 15.8km, Climb: 949m, Area: Stradbally Mountain, Central Dingle (Ire

  
Summit Summary
Seefin Mountain W Top: Bog and rough ground.
Collaborative entry Last edit by: melohara 2 days ago.
Car park space at R66276 16811. Over or around the gate and head uphill on a good track that crosses the hill near the summit of Seefin Mountain East top.Follow the track on downhill from the cres...

  
Track
Pic du Midi d'Ossau
David-Guenot a day ago.
The idea came from my 60-year-old, freshly retired neighbour, as we and some other members of the local badminton club m walk, Len: 16.2km, Climb: 1353m, Area: France, Nouvelle-Aquitaine ()

Summit Summary
Seefin Mountain E Top: Up the old bog road.
Collaborative entry Last edit by: melohara 2 days ago.
Car park space at R66276 16811. Over or around the gate and head uphill on a good track that crosses the hill near the summit.On the crest of the hill the summit cairn is over a wall to your right...

  
Summit Summary
Seefin South-East Top: Seefin South East Top is easily visited
Collaborative entry Last edit by: melohara 2 days ago.
Seefin South East Top is easily visited from the track leading to Seefin Mountain East Top and Seefin Mountain West Top which starts st R66276 16811. Just follow the track to R65202 17892 from whe...

  
Summit Comment
Carnaween: The visitors book box near the summit
Aidy 6 days ago.
Just wanted to show the brilliantly constructed little box which contains the visitors book. It was too wet and windy on my visit to take it out and risk ruining it. A few shafts of light breaking...

Summit Comment
Carnaween: Bad Weather Adding Drama To Great Views
Aidy 6 days ago.
I used the commonly taken route from Disert Graveyard (worth a visit in its own right) over ground that was alternatively boggy or pretty steep. It was freezing cold in these parts, with a strong ...

  
Track
Pic Rouge de Bassiès
David-Guenot 2 days ago.
This was the first of the two big hikes I did in the Pyrenees in summer 2016. Hiking in the summer often means a sunny d walk, Len: 16.3km, Climb: 1342m, Area: France, Midi-Pyr?n?es ()

  
Summit Comment
Corrin: A family stroll.
David-Guenot a week ago.
Climbed this hill in November 2014, with a friend who used to live in the area. Starting at the Coillte car park, we did a nice loop on the forest paths, circling around the hill in an anticlockwi...

Forum: General
MV Mountain Gathering 2017
Onzy 4 days ago.
Always a great night ... all welcome.

  
Summit Comment
Luggala: Fancy View from White Hill
davsheen 2 weeks ago.
Autumn's evening view across to Luggala/Fancy from White Hill on route down from Djouce using the Wicklow Way boardwalk section.

  
Summit Comment
Binn Charrach: The peat-hag capital of Connemara
markwallace 2 weeks ago.
The descent from Benbaun to the R344 via Knockpasheemore/Binn Charrach is hard going after a day in the Bens. The Knockpasheemore ridge looks nice and flat from below, but is boggy, undulating and...


RECENT CONTRIBUTIONS 1 2 3 .. 17 Next page >>