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Underwater Visibility

Underwater Visibility

Visibility is an estimation of water’s clarity and is described as the distance a diver can see horizontally. Most divers abbreviate visibility colloquially as “viz,” and it is given in units of distance such as “sixty feet of viz.”

WHAT FACTORS AFFECT VISIBILITY

Commonly listed factors which affect visibility include suspended particles, weather, and water movement. However, these factors seem like one factor as they are all somewhat linked to the weather condition. Below is the list of the common factors which affect underwater visibility:

  • SUSPENDED PARTICLES

Suspended particles of sand, clay, mud or other bottom sediments affect underwater visibility pretty much the same way fog affects visibility on land. Visibility reduction caused by floating particles may be severe depending on the type, density and amount of suspended particles in the water.

For example, clay sediments quickly get suspended, reducing visibility to nearly zero feet in a few moments and remains in suspension for several hours. On the other hand, sand does not get suspended as easily as clay, rarely reduces visibility to zero and falls out of suspension in a couple of minutes.

Sediment particles become suspended once disturbed by water movement or by divers. Natural causes of water movement which force particles into suspension include wave action, currents, choppy seas, runoff and rough weather. A diver can stir up bottom particles by using improper kicking techniques, by swimming with his hands or landing on the bottom.

  • SALINITY

Water of varying salinities forms distinct layers like that of vinegar and olive oil. The interface between both layers is called a “halocline,” and when it’s viewed from above, it resembles a shimmering underwater river or lake. However, when water of varying salinities is mixed, visibility is reduced. Loss of visibility underwater can be extreme- a diver can see light but can’t distinguish shapes and in some cases have difficulty reading his gauge.

Haloclines are often encountered in estuaries, at springs which empty into the ocean, and at inland caves and caverns. A diver may notice the blurry effect of mixing of fresh and salt water near the ocean’s surface during a storm, as fresh rainwater mixes with the ocean’s water.

To avoid the visual disturbance which a halocline causes, a diver must swim below or above the depth where water at different salinities mix. Once a diver exits this mixing region, the visibility gets cleared immediately. If descending or ascending to escape the halocline is possible, a diver can reduce visual disturbance by swimming to the side of other divers but never behind as their leg flipping motion will make the visual disturbance worse.

  • TEMPERATURE GRADIENTS

Thermocline represents a temperature gradient or a level at which two different temperatures meet. The effects of temperature on visibility are similar to the effects of salinity. Cold water is denser than warm water, and sinks below. Therefore, divers typically experience increasingly cold layers as they descend. When the temperature difference between both layers is highly significant, the interface between both layers looks “oily.” In general, the visual disturbance which is created by different water temperatures is not significant, and a diver swiftly passes through the thermocline region as he descends or ascends.

  • ORGANIC PARTICLES

Alga or bacteria blooms can affect visibility in a very dramatic way. A typical place where this sort of visual disturbance is encountered is in a fresh water body with little or no water circulation. Algae and bacteria require very particular temperature, salinity, and light conditions, and may occur only seasonally. The presence of alga or bacteria blooms forms an opaque, greenish cloud which may extend as far as 5 feet deep into the water. Divers must descend through the opaque cloud in near zero visibility before getting to the crystal-clear water.

  • HYDROGEN SULFIDE

Often, a diver is likely to encounter hydrogen sulfide, and it is probably found in fresh water bodies with little circulation where putrefying organic matter is present. When only a small amount of hydrogen sulfide is present, it forms thin, smoke-like wisps. Here, the visibility is almost zero.

 

Visibility is affected by some factors. Identifying the cause of a visual disturbance allows a diver manage it properly. Visual disturbance may be caused by factors aside water clarity, such as foggy masks, narcosis, reduction of ambient light and oxygen toxicity. The cause of reduced visibility must be identified by the diver and a decision made on whether to continue the dive or not.

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