What is visibility?

[ScubaRob0311]

Silly question but how is visibility measured underwater and what types of things effect visibility in freshwater? Are there certain things you can look for to gauge how visibility might be before you get to dive site?

 

[archman]

Some of us have twisted around the old school secci disk principles as a crude way of measuring underwater viz.

We run a white or a black or white/black disk sideways in the water column, and then see how far away we can drag it away until we can't see it no more.

 

[wetb4igetinthewater]

Most people figure it to be how far they can see. This is fairly coarse. A more accurate measurement is with a Secchi disk as mentioned by archman. The distance in which you can make out the black and white pattern is the visibility distance. Secchi disk - Wikipedia

 

[ScubaRob0311]

Ok so let's say an area has visibility of 10 feet. Would that mean at 10 feet I wouldn't be able to see a fellow diver or could I see them but not make out much detail?

 

[archman]

They might be a murky blob at 10 feet. At what distance one can still recognize a diver profile vs. a "blob" is a great philosophical debate in the general diving community.

Look at the guy in the photo holding the secci disk in the pic I posted above. Is that guy a blob or not? Picture him now in a black wetsuit. I'd coin flip with calling him a blob.

 

[lowviz]

Silly question but how is visibility measured underwater

Not silly, complex.

In good viz, you tend to have a lot of light. In poor viz, you have much less but your eyes adapt. Just imagine that bad viz falls off faster than good viz. Good enough approximation to keep buddies together.

 

[ScubaRob0311]

They might be a murky blob at 10 feet. At what distance one can still recognize a diver profile vs. a "blob" is a great philosophical debate in the general diving community.

Look at the guy in the photo holding the secci disk in the pic I posted above. Is that guy a blob or not? Picture him now in a black wetsuit. I'd coin flip with calling him a blob.

I can easily identify him as a diver in that photo but I think I would have to study image more if in a full black wetsuit.

 

[archman]

My wife mistook a manatee for an "ugly dolphin" once. I would call that a sighting at the extreme outer range of Viz, ha ha.

 

[Ghost95]

I always find it funny when someone says they had only 3' visibility. I ask them if they could see their fingers when they stretched out their arm. They're surprised when they realize that the viz was probably quite a bit better than 3'.

Visibility is very subjective with new divers. The estimates are always interesting.

 

[ScubaRob0311]

I always find it funny when someone says they had only 3' visibility. I ask them if they could see their fingers when they stretched out their arm. They're surprised when they realize that the viz was probably quite a bit better than 3'.

Visibility is very subjective with new divers. The estimates are always interesting.

That's exactly why I'm trying to get in sync with reality lol