Factors That Affect Visibility Underwater When Diving

Visibility is an estimation of water clarity and is defined as the distance a diver can see horizontally. Many divers abbreviate visibility with the slang term “viz.” Visibility is given in units of distance, such as “fifty feet of viz.”

PADI's review questions from the open water course review several main factors that affect visibility underwater: weather, suspended particles, and water movement. These seem like only one factor to me, as weather causes water to move, which causes particles to float into the water. Here is my list of five common factors that can disturb visibility underwater.

Suspended particles of sand, mud, clay, or other bottom sediments effect the visibility underwater in much the same way as fog effects visibility on land – distant shapes become colorless, poorly-defined shadows. Visibility reduction caused by suspended particles may be slight or severe depending upon the density, type, and amount of sediment suspended in the water. As an example, clay sediment will become suspended easily, will reduce the visibility to nearly zero feet in a few moments, and will remain in suspension for many hours. In contrast, sand does not become suspended as easily as clay, rarely reduces the visibility to zero, and will fall out of suspension in a matter of minutes.

Sediment particles become suspended when they are disturbed by water movement or by divers. Natural causes of water movement that forces particles into suspension include currents, wave action, choppy seas, runoff, and rough weather. A diver can stir up bottom sediments and reduce visibility by using improper kicking techniques, by swimming with his hands, or by landing on the bottom (one of the many reasons these actions are discouraged).

Water of different salinities forms distinct layers in a manner similar to that of olive oil and vinegar. The interface between the two layers is called a “halocline” (halo = salt, cline = gradient). When viewed from above, an undisturbed halocline resembles a shimmering underwater lake or river (an effect caused by the variation of refractive properties with salinity). However, when water of different salinities is mixed, the visibility becomes very blurry. Divers have compared the visual effect of swimming in a disturbed halocline to having lost one contact lens, to being inebriated and unable to focus, and (my favorite) to swimming in Vaseline. The loss of visibility in a halocline may be extreme; a diver can see light but cannot distinguish shapes. In some cases a diver in a halocline may even have difficulty reading his gauges! See a photo taken through an undisturbed halocline.

Haloclines are encountered in estuaries, at springs that empty into the ocean, and at inland caves and caverns. A diver may also observe the blurry effect of mixing fresh and salt water near the surface of the ocean during a rainstorm, as the fresh rainwater mixes with the ocean's saltwater.

To avoid the visual disturbance caused by a halocline, a diver must swim above or below the depth where water of different salinities mixes. Once a diver leaves this mixing region, the visibility clears immediately. If ascending or descending to escape the halocline is impossible, a diver can minimize visual disturbances by swimming to the side of (but never behind) other divers, as their kicks will mix the water and make the visual disturbance worse.

The term “thermocline” signifies a temperature gradient (thermo = temperature and cline = gradient), or a level at which water of two different temperatures meets. Water of different temperatures layers similarly to water of different salinities, although the effect is not as pronounced. Colder water is denser than warmer water, and sinks below it. Therefore, divers will typically encounter increasingly cold layers as they descend. When the temperature difference between two water layers is extreme, the interface between the two layers looks “oily” (similar to a halocline). In general, the visual disturbance created by different water temperatures is not great, and a diver quickly passes through the thermocline region as he ascend or descends, hopefully enjoying the pretty visual effect.

Bacteria or algal blooms can disturb the visibility in a very dramatic way. A typical place to encounter this sort of visual disturbance is a body of fresh water with little or no circulation. Algae and bacteria usually require very specific conditions of temperature, salinity, and light, and may be present only seasonally. An example is Cenote Carwash in Mexico's Yucatan Peninsula, where an algal bloom is present only during the warmer months. The algal bloom forms an opaque, greenish cloud extending from the surface to about 5 feet. Divers must descend through the cloud in near zero visibility before reaching the crystal-clear spring water of the cenote. The presence of organic particles may also be indicative of pollution. See a photo of diving in Cenote Carwash.

Unless he is diving in a cave or cavern, a diver is unlikely to encounter hydrogen sulfide. Hydrogen sulfide is most commonly found in fresh water with little circulation where decaying organic matter is present. Large quantities of hydrogen sulfide tend to form a dense, foggy layer, as in Cenote Angelita in Mexico. When only a small amount of hydrogen sulfide is present, it forms thin, smoke-like wisps. Inside a cloud of hydrogen sulfide, the visibility is almost zero. Hydrogen sulfide is worth mentioning because the visual effect is fascinating. Check out a photo of the hydrogen sulfide cloud in Cenote Angelita.

Water clarity, or visibility, is affected by a variety of factors. Identifying the cause of a visual disturbance will allow a diver to manage it correctly. Keep in mind that visual disturbances may be caused by factors other than water clarity, such as foggy masks, reduction of ambient light, nitrogen narcosis and oxygen toxicity. The cause of any reduction in visibility or visual disturbance should be identified by the diver, and proper judgement should be used when deciding whether to continue with the dive or not.

_{More important information about visibility and scuba diving:}

• Avoid problems associated with diving in clear and turbid water.