Judging Visibility

[mccabejc]

I think I tend to be pessimistic when I judge visibility, and I'm curious how you guys do it. I figure that if you can see enough detail to identify the fish you're looking at, including colors, then that's visibility. If all you can see is a silhouette, then that's not visibility. If you can only see a big patch of dark in front of you where a reef is, that's not visibility. But if you can see details of the reef, and colors, then that's visibility.

Also, there's viz and then there's viz. You can have a clear 15-20 ft., or a murky 15-20 ft. I guess if it's murky, then I tend to scale back my estimate. Just because.

 

[Al Mialkovsky]

If I look back and can see my fins I know I am not in Oregon.

 

[Divemastr]

My 2 cents on viz... The distance where I can see the student's outline CLEARLY is the viz. I may not be able to see the the facial expressions but I can tell where they are and it is a bit more than a silhouette. I can see the outline of the student's limbs, etc. In Cozumel, we were searching for a camera that someone lost at the safety stop. We could see it from the surface on the sand at 80 feet. It was more than just the outline. I could see the strobe clearly. It was small from that distance, but was visible. I love diving Cozumel.... Just had to say that...

 

[DA Aquamaster]

I remember reading that visibility was officially measured (by someone - don't ask me who) using a 4" diameter white plate. The distance at which you could still discern the plate at all was considered the max visibility.

In practice, I go with the distance at which I can discern what an object is. Outlines and darker or lighter spots in the water are normally visible a bit farther, but I don't count that. I have also noted that vertical visibility is often much better than horizontal visibility, or vice versa if you have different thermoclines and layers involved.

 

[Rick Inman]

I have also noted that vertical visibility is often much better than horizontal visibility, or vice versa if you have different thermoclines and layers involved.

Yeah. In warmer months, in local waters, the first 70 feet may have 10' horizontal vis and 25' vertical. Then, when you hit the thermal below 70, the horizontal jumps to 25' and the vertical drops to 10'. Or so it seems.
Can anyone explain how this happens?

 

[pipedope]

Visability means that I can see something.
Great vis means that I can see it before I can touch it.


I think the tool is a 'Sechi disk'. I don't know if the spelling is right. The disk is marked in quarters alternating black and white.

 

[Don Burke]

http://sebagolakeassc.org/secchi.htm

 

[gcbryan]

Yeah. In warmer months, in local waters, the first 70 feet may have 10' horizontal vis and 25' vertical. Then, when you hit the thermal below 70, the horizontal jumps to 25' and the vertical drops to 10'. Or so it seems.
Can anyone explain how this happens?

Near the surface you have sunlight improving the vertical viz. Down deeper you don't. Horizontal viz is bad in the summer near the surface due to algae (due to sunlight). Horizontal viz improves when there is less light with depth.

Am I missing something in your question?
Gray

 

[String]

If i can see fin tips of buddy and their head its roughly 2m. If i can sww twice their length its about 4m.

If i have to hold my computer against my mask to see it then its about 2 inches

I do generally use buddy lengths to judge vis though as its a good measurment in the middle of the scale vis i tend to dive in.

 

[ShakaZulu]

So let me try to understand this, the Viz is about as far as you can see underwater, wow, what a concept.......