Swampdogg:
In perfectly gin clear water, like bottled water clear, how far could you see? I think 100' would be the max. I understand depth and light absorption play a role.
I find that there is a huge discrepancy in people's estimation of what the viz is on any given day. What would you say the end of the visibility is determined by? If the viz was 40', could you see a diver at 45'? I usually will estimate the viz to be the distance that I can see clearly. So if I estimate the viz to be 20', I usually can make out objects and divers at 30-35', but fuzzy.
I started thinking about this in the pool yesterday. One length is 25 yards (75 feet) and I could not make out the markings on the far side of the pool horizontally (With a mask on, of course). When I hear that the viz is 100', I find that hard to believe.
I am very used to diving limited viz inshore water, and I find that when others think the viz is awful, I think it is pretty good.
Interesting question.
Before I forget, here's a link to some reports of Antarctic diving where the viz was estimated at 500 to 800 feet! There's an interesting picture on one of the pages that shows an ice wall and the ocean bottom where the ice sheet "grounded". Turns out the wall is about 120' tall and the little bright speck in the middle is the light of a diver.... perhaps hundreds of feet from the photographer!
http://scilib.ucsd.edu/sio/nsf/gallery/gallery3.html
I have a simple answer to how I define viz: it's the distance beyond which an object cannot be seen. That would be horizontally, unless otherwise specified.
For my practical purposes, the object doesn't have to be seen clearly, just recognizably.
For example, I might just barely make out a buddy's brightly colored fins, but not his dark drysuit from 30 feet away. I'd say the viz was about 30' at that depth.
Anyway, I like to actually measure the viz when it's real good, so I have my flag line marked every ten feet. Tied to an object, I can swim away until the object just disappears from view, then swim back until it reappears. That's the viz, in my book.
It can be very surprising. I once measured viz of 75' horizontally off a beach in Rockport, MA. I wouldn't have believed it if I hadn't measured it!
Vertical or diagonal viz is almost always going to be better than horizontal, so I only occasionally will make a mental note, such as that the surface waves could be seen from 80', for instance. It's still a nice recollection to have in a log book.
I remember wondering in Bonaire how good the viz was, but all I had to go on was vertical, since I didn't have a marked reel or flag line with me. Vertical one day was in excess of 130'.
Florida springs might be over 200', but I've only estimated.
It's amazing how quickly viz can change on the same dive, too, right? There might be pockets of clearer water and others that are pretty soupy, perhaps because currents drag some surf-zone water over there.
Then there are thermocline layers below which suspended particles might obscure viz.
Sorry for going on so long, but I once found an interesting layer that surprised me near the deep hole of a local lake.
Viz was about 30' at depths down to 60', then viz gradually dropped to about 5' until reaching 104' deep. Then there was a clearly defined layer that looked like thick smoke. I stuck my light into the smoke and it disappeared when at arm's length!
I ventured a few feet into that stuff and then decided I really didn't need to get to the 110' max depth of that lake after all!
Needless to say, I was very happy to get back to the 5' viz!
Dave C
