Defining visibility

[homerdoc]

Interesting question from my daughter filling her dive log after a dive.

She asked me what the visibility was today and I told her 30 feet ish.

She then asked me how I determined that - I was stumped - I just said I thought I could clearly make out stuff at 30 feet or so away.

Is there an actual technical definition of visibility?

I'm attaching a picture from the dive.

 

[Robert H. Diver]

I just guess LOL.

 

[CycleCat]

It's always an estimation. My dive logs usually say 30-50-100.

 

[RIHappyDiver]

I struggle with judging distance, so i think “how many of me would it take to get to the furthest I can see?” Even then dive buddies will differ with me. Now, if I look under me and can’t see the tips of my feet? Well, that’s easier. I always get a kick when some tells me a distance that doesn’t end in a 0, as in 23’ vis.

Now, to answer your question, none that I know of.

Erik

 

[TheCoolBeans]

In the UK we have some ‘challenging’ conditions on occasion.

I’ve previously written down:
“Computer screen hazy with low viz”

And…

“Couldn’t see computer screen until arm bent”

Most of the time viz is 2-3m, I can buddy dive with 3-4 others but more than this Diver1 can’t see Diver5 etc.

If I’m following behind you and I can see your bubbles I’m calling that 2m /6ft viz

If I can see the diver ahead of you then that’s another 2m/6ft etc etc.

Another trick is to check depth and look to the surface, what can you see?

 

[wetb4igetinthewater]

My understanding it is measured by how far a Secchi disc can be made out (Secchi disk - Wikipedia).
More here: Secchi disk depth: A new theory and mechanistic model for underwater visibility

 

[homerdoc]

I know what a dumb question - I am with you on the depth thing (my daughter is in college to be an engineer so she is having some fun questioning my 35 year old engineering analysis)

But - We dove in 80 foot of water and cold see the bottom clearly at 40-50 feet down as we dropped - I think this is my new technical definition.

Thanks for all the responses - she laughed at the just guessed - she told me that I is what I was doing!.

 

[gmerick]

Most of the time I use my depth gauge on my computer. I check the depth when I can see detail on the bottom, then check again when I get there. With low viz, I look at (look for??) other divers. if I'm 1m behind her fins, and I can't make out her mask strap 3m viz.

 

[homerdoc]

That is outstanding - thank you for the response @wetb4igetinthewater

 

[Eric Sedletzky]

I always judge it by how far away something completely disappears. Where I dive we need all the help we can get!
If I judged it by when something begins to diminish the vis would only be about two feet.