Defining visibility

[wetb4igetinthewater]

That is outstanding - thank you for the response @wetb4igetinthewater

You are most welcome. And this is a very valid question. While I do own a secchi disc, I don't use it on every dive, only for evaluating training sites. I use the similar definition as others: estimating how far I can see when things degrade to where I can't make it out. I have no idea how accurate it is. In the Puget Sound, I'm happy if I can see 15 feet or more away. So when I am diving in the tropics and people complain about bad viz of "only" 50 feet, I just chuckle inside.

 

[Bob DBF]

I have no idea how accurate it is.

If not accurate, at least consistent, since it's been used since 1865 to determine transparency (visibility) of water.

 

[wetb4igetinthewater]

If not accurate, at least consistent, since it's been used since 1865 to determine transparency (visibility) of water.

I was referring to my estimation when not using the secchi disc. That is the most accurate.

 

[tursiops]

The Secchi disk was developed to estimate light penetration to depth, for the purposes of estimating biological productivity. It was a 19th century concept no longer used in science. Worse, vertical visibility is not much related to horizontal viz. Diver (horizontal) viz is best estimated by seeing how far away you distinguish a black object. There is a long literature on this in the field of optical oceanography.

 

[Brett Hatch]

In my experience, it seems like horizontal viz is always a bit worse than vertical viz. Maybe by a factor of 1.5, something like that. So a day of 30' horizontal viz might have 50' vertical viz. @tursiops do you happen to know why this is? I cannot think of a scientific basis for it... maybe it's just a psychological phenomenon, like some sort of selection bias.

Edit to add: @wetb4igetinthewater Kosta, have you measured vertical viz vs horizontal viz in the same conditions with the secchi disk?

 

[NW Dive Dawg]

If I can see my fins then the vis is 6-7 feet. If I can't see my fins then I just I head for the pub where I can clearly see my drink.

 

[Bob DBF]

In my experience, it seems like horizontal viz is always a bit worse than vertical viz. Maybe by a factor of 1.5, something like that. So a day of 30' horizontal viz might have 50' vertical viz. @tursiops do you happen to know why this is? I cannot think of a scientific basis for it... maybe it's just a psychological phenomenon, like some sort of selection bias.

Edit to add: @wetb4igetinthewater Kosta, have you measured vertical viz vs horizontal viz in the same conditions with the secchi disk?

I don't think it's psychological, it may have to do with the way the light penetrates the water vertically, but wtf do I know.

 

[wetb4igetinthewater]

In my experience, it seems like horizontal viz is always a bit worse than vertical viz. Maybe by a factor of 1.5, something like that. So a day of 30' horizontal viz might have 50' vertical viz. @tursiops do you happen to know why this is? I cannot think of a scientific basis for it... maybe it's just a psychological phenomenon, like some sort of selection bias.

Edit to add: @wetb4igetinthewater Kosta, have you measured vertical viz vs horizontal viz in the same conditions with the secchi disk?

I haven't, but it would be s good exercise. As viz varies by depth, it would be interesting to measure viz at the surface versus 20 feet vertically and horizontally at the same depths. A local dive site Cove 2 has buoys that I could attach the disc. I could put my camera in manual mode to have consistent pictures

 

[Landau]

One of my buddies just records visibility as emojis. Something like this from good to bad:



In my local dives visibility often varies dramatically by depth or at different parts of the dive site. I had visibility of about 1 foot from 0 to 35 feet deep and visibility of about 80 feet below 40 feet deep - where it was pretty much a night dive in the middle of the day, all the night critters were out at noon.

 

[rob.mwpropane]

If you find yourself in calm water its always fun to look at the other person on the surface and then look at their lower 1/2 just to see how distorted water makes our vision. I think water reduces the amount we see by 1/3?, so when you look in a fish tank you only perceive 2/3's of the real-estate that's really there. Jump to the side and see how "deep" it really is. Should be a sign on masks "objects are father than they appear", lol.

That is my best guess at vertical viz and I know about how far away I am from the person just because of too many years with a tape measure.