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Just How far is that?

Just How far is that?

One of the greatest tests of diving endeavour is to stay out of the water during bad weather, not just for your safety but also your enjoyment.

 

I appreciate divers want to go diving as often as possible, but when the local weather turns nasty and does not allow us to dive. We have to just, sit back and wait for the bad weather to clear. Sometimes desperation steps in, common sense departs, bravado takes over and this is the time that problems do occur and divers do, get injured.

 

 

One of the most important considerations for diving is the visibility, "vizo" or “viz” whatever you want to call it. Visibility especially very good "viz" or extremely clear water will make any dive site seem unreal, on the other hand, I remember a dive at Steves Bommie on the Great Barrier reef in '98 we had 2 to 3 metres viz, this was terrible visibility for a divesite that is usually a great dive. 

 

 

So what is visibility? Visibility is generally considered to be the distance at which an object underwater can be clearly identified. 

 

Underwater visibility is measured three ways. 

 

Horizontal visibility which is the distance you can see looking straight ahead - 
Vertical visibility which is the distance you can see looking directly up or down.

 

 

Horizontal visibility is usually more important, since it affects our ability to view the amazing underwater environment and keep track of our fellow dive partners.

 

The main factor that primarily affects underwater visibility is light penetration along with floating particulate matter. Not surprisingly, these factors are often related to one another, as well as to other environmental factors. 

 

 

LIGHT PENETRATION
The amount of light penetrating the waters surface depends on surrounding ambient light levels (cloudy versus sunny day), also the angle at which the light rays meet the water plus the roughness of the surface. 

 

SEA STATE 

 

Sea state is another factor affecting light penetration. Rough seas reflect more light, thus reducing overall underwater illumination. 

 

So if you’re choosing between two dive sites, opt for the one with calmer less choppy surface water. The visibility will be better. 

FLOATING PARTICULATE MATTER
The amount of floating particulate matter in the water depends on the degree of water movement and the local sea bed state. 

 

Wave action and storm surge stir up the bottom sediment and lifts this lighter particulate matter up into the water column. The size and type of particulates, determines the time required for them to settle back onto the bottom again. 

 

Coarse or heavy grains settle out quickly, whilst fine sandy silt will take much longer. So if you’re thinking of diving in an area a couple of days after a big storm, opt for a dive site that has a rock or gravel bottom as opposed to one that has a fine sand, silt or mud bottom.

 

 

The way I measure visibility;
As I descend, I determine vertical visibility by noting the full disappearance of the surface and cross-referencing it to my depth. 

 

Halve that depth number and I have the approximate horizontal visibility. Remember, you are looking towards an area of greater light source and can see further, looking up. 

 

In other words, if vertical visibility is approximately 20 metres then the horizontal visibility is around 10 metres.

 

 

Another measure I use is kick cycles. I find an object right on the edge of my vision, swim towards it counting my kick cycles, do the math and there I have a fairly good measurement.

 

 

Next time you go for a dive, ask divers “How was the viz ?” 
I guarantee you will get a wide and varied response.

 

 

 

 

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