Maybe dumb ? about visibility

[Arkman]

Ok, this maybe is a really dumb question but oh well.

When your diving how can you tell the visibility? I understand that its as far as you can see. But how do you estimate that in feet (or whatever unit of measure). I have terrible sense of distance so I'm not sure short of taking out my tape measure how far it is.

I guess what I'm asking is do you all just estimate and say "oh that looks about 10 feet away" or is there some good way to judge the visibility that I'm not aware of?

 

[Mike Boswell]

It's an excellent question, and I think most divers just guess. If your buddy is one buddy length away from you and you can just barely see her, then the vis is about 5 feet. Distances much farther than 25 feet are hard to estimate accurately - in water and on land too - and different people will get widely different numbers. When I hear people say the vis is 80 or 100 or 120 feet, I just take that to mean the water was "pretty clear".


In San Diego, the vis often changes markedly with depth, so there is no one number that describes it. Dive reports often say that the vis was 15 feet down to a depth of 60 feet but then opened up to 40 feet below that. I have even seen big changes in vis over the course of a single dive, as the tide flows in or out.

Sometimes, from the bottom you can see the water surface or the bottom of your boat, and you know from your depth what the minimum vis is. Sometimes you can see the bottom from the surface, and often when you are descending, the bottom will slowly come into view, and from your own depth and the bottom depth you can figure the vis that way as well. Note that these measures are vertical vis, which is different than horizontal vis.

To measure visibility accurately, engineers use a thing called a Secchi disc, which is a black and white disc lowered into the water from the surface on a rope. When you can no longer see the disc, you measure the length of rope and that is your visibility. Secchi disk - Wikipedia, the free encyclopedia Scientists use an instrument called a tranmissometer which uses a light source and meter.

 

[oldnotdead]

Ask as many people as you want to that were in the water on that dive and compare to your own thoughts.

 

[redacted]

In TX we estimate visibility the same way we weigh a hog.

You get a big long plank of wood and a pile of rocks. You pick out a good round rock and place it under the middle of the plank. Then you tie the hog to one end of the plank and start piling rocks on the other end until it balances. Then we estimate the weight of the rocks.

 

[Krenath]

Since I have a depth gauge with me, I tend to use it to get an idea of depth if the water's deeper than the visibility distance.

If I'm on a wreck that I know is at 100' and while I'm surfacing, the details of the wreck get hazy it's nearly lost in the blue when I'm at 30' then I know the visibility is around 70'.

If I'm in really shallow water and people on the boat can easily see the reef and the sand, that's not going to help, but having seen what visibility of 70' looks like on something like the above-mentioned wreck, you can compare the visibility on the reef and get an idea about how far.

I also sometimes imagine easy-to-visualize objects of known size for reference. "If our 65' boat were to sink, would I be able to see both ends?" as an example...

 

[Mike Boswell]

In TX we estimate visibility the same way we weigh a hog.
You get a big long plank of wood and a pile of rocks. You pick out a good round rock and place it under the middle of the plank. Then you tie the hog to one end of the plank and start piling rocks on the other end until it balances. Then we estimate the weight of the rocks.

How long does the plank have to be to get an accurate measurement?

 

[Arkman]

In TX we estimate visibility the same way we weigh a hog.

You get a big long plank of wood and a pile of rocks. You pick out a good round rock and place it under the middle of the plank. Then you tie the hog to one end of the plank and start piling rocks on the other end until it balances. Then we estimate the weight of the rocks.

I actually laughed out loud at my desk at work. That was funny.

Since I have a depth gauge with me, I tend to use it to get an idea of depth if the water's deeper than the visibility distance.

If I'm on a wreck that I know is at 100' and while I'm surfacing, the details of the wreck get hazy it's nearly lost in the blue when I'm at 30' then I know the visibility is around 70'.

If I'm in really shallow water and people on the boat can easily see the reef and the sand, that's not going to help, but having seen what visibility of 70' looks like on something like the above-mentioned wreck, you can compare the visibility on the reef and get an idea about how far.

I also sometimes imagine easy-to-visualize objects of known size for reference. "If our 65' boat were to sink, would I be able to see both ends?" as an example...

That seems like a good way. Not that I have to worry too much. I would guess the places that I dive the vis is no more the 5', maybe 10 on a good day.

Thanks for the advice...and the laugh.

 

[Mike Boswell]

Here is a pretty good article about vis. What is visibility?

 

[TSandM]

To the OP: The fact is, most visibility reports are a little better than WAGs. If the visibility is really poor, it's pretty easy to estimate -- if I can't see my hand in front of my face, it's less than a foot. If I can't see the bottom until I hit it, it's down there, too.

I use a personal system. If I have to swim with my nose in the sea lettuce to see the bottom, viz is 2 to 3 feet. If I can dive normally but the team has to stay within bumping distance, it's 10 feet or less. If we can dive normally and spread out a bit, it's in the 15 foot range. If I can see navigational landmarks well before I get to them, it's in the 25 foot range. And if I get in the water and go, "Wow, this is GREAT viz!" it's better than 25.

 

[String]

As said, people guess visibility and often wildly inaccurately. The better the visibility the more inaccurate the guesses.

Generally if i can see my buddys head and his fins i know its about 2m. Judging 1m,3m,5m is a lot easier than judging the difference between 20m and 30m and 40m etc.

Also horizontal visibility can differ substantially from vertical.