Define "visibility"

[PnL]

If one states that there is 10' of visibility does this mean that they can see an object at 10' with detailed clarity, or just being able to discern its identity? I.E., If I can see my buddy's yellow fin's through the murk at 10' but can't see him would I say the visibility was 10'? Or would I have to be able to see him clearly?

We were at Laguna this weekend and the visibility sucked. I think the visibility was 2-4' but my buddy thinks it was 5-10'. I could see the outline of rocks at around 4' but could not see anything on the reefs until I got much closer ... around 2'. What was the visibility during this dive?

TIA
Paul

 

[Rick Murchison]

The only definitive and useful (to us as divers) method I'm aware of is with the use of a secchi disk. Originally designed to measure vertical visibility, it can be applied to horizontal vis as well.
See http://www.mlswa.org/secchi.htm
Rick

 

[PnL]

Thanks for the link.

But short of doing a scientific survey each time I dive, how would I define visibility in the instances given in the initial post?

Paul

 

[jiveturkey]

I 'm interested in what people have to say on this one to. I have trouble telling people what the vis is like. I usually say good or bad or something but I don't think I could tell anyone in terms of meters.

 

[Rick Murchison]

I try to describe vis in terms of how far away I can see my buddy. (Having played with a secchi disk it turns out that's about the same thing). And until you get a little practice measuring (actually measuring - with a line or tape measure or some other objects a known distance apart) you're most likely not very good at it.
As for "good" or "bad" visibility, that depends on what you're doing. If you want to look at pelagics, 50 feet may be lousy - if you're looking for artifacts in a river then two feet may be exceptionally wonderful.

What I do to make every dive a great dive is to bring my focus in to suit the visibility... if the vis is only two or three feet, then I concentrate on the little guys - the worms & crabs & snails & clams & such. I remember one particularly memorable dive one night when my buddy and I spent over an hour on a single ten foot beam of a bridge span, absolutely captivated by the living show there. We both came back aboard the boat all grins and saying corny things like "awesome" and "that was just *fine*" and "did you see that one decorator crab with the green stuff on his feet too?", while the rest of the folks were whining about the poor vis and looking at us like we'd lost our minds.
Rick

 

[trymixdiver]

Visability implies that you can see something.

Low vis, poor vis, no vis , average vis

I also dive laguna. I would say our average vis is 10-20'. That said, i would call it Average. On them really crappy days low or poor vis i would say 5-10' . And for those really fun days at laguna no vis or just your hand infront of your mask, which we dont get that much.

Andy

 

[Dee]

ScubaKims once bubbled...
If one states that there is 10' of visibility does this mean that they can see an object at 10' with detailed clarity, or just being able to discern its identity? I.E., If I can see my buddy's yellow fin's through the murk at 10' but can't see him would I say the visibility was 10'? Or would I have to be able to see him clearly?
TIA
Paul

I go by whether I can tell what a subject is. ie...Is that black shadowing thing a diver or am I swimming under a tall patch of grass? I don't need to distinquish a divers features, just know it's a diver. 'Course there are times when I know the yellow blur in front of me is a diver but only because I know someone with us has yellow fins!

So my 'vote' is just discerning identity.

 

[Big-t-2538]

Being able to identify an object....Not being able to read something on it. Say a no parking sign underwater. If I could see it is a sign, but not read the wording, I would consider that distance the range of viz.

 

[PnL]

Thanks for the replies everyone - it clears things up a bit. I probably will not be bringing a measuring tape diving with me in the near future but at least I have a definition of what 'visibility' means.