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What is visibility? Illuminating facts about an unclear situation

By Robert N. Rossier

What’s the difference between a great dive and a mediocre dive? In most cases, it’s a matter of feet — not in terms of depth, but underwater visibility.

One of the most important considerations for diving is the “viz”. Clear water can make a mundane dive site shimmer in glory. Conversely, even the best coral reef can be a disappointment when it’s clouded by a veil of murky water.

Visibility is generally considered to be the distance at which an object underwater can be readily identified. Underwater visibility is measured two ways. There is horizontal visibility —how far you can see looking straight ahead — and vertical visibility — how far you can see looking up or down.

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

Despite its importance, visibility receives precious little attention in a basic scuba course.

That’s too bad because if you know something about the factors that affect visibility, you’ll

have a better chance of locating clearer waters.

Three factors primarily affect underwater visibility: light penetration, biological species and particulates. Not surprisingly, these factors are often related to one another, as well as to other environmental factors.

LIGHT PENETRATION

It only takes a couple of dives before we realize that sunlight plays a very important role in the visibility equation. Just as we can usually see better on the surface during a sunny day, the same generally holds true underwater.

The amount of light that will penetrate the water depends on three things: overall light levels (i.e., a cloudy versus a sunny day), the angle at which the light rays meet the water (called the “angle of incidence”) and the roughness of the surface.

While you may not be able to do anything about the clouds, you can plan your dives to take advantage of optimum light levels. In a geographic sense, when you’re near the equator,

where the sun’s rays are most intense, more light penetrates the water surface. In higher latitudes, the lower angle of incidence causes more light to be reflected, and less enters the water column. That’s one of the reasons why the water is almost always clearer in Bonaire (which is very close to the equator) than it is off the coast of New Jersey.

The location of the sun in the sky also affects light penetration, with the best light levels occurring around noon. The further the sun sinks on the horizon, the smaller its angle of incidence and the worse the visibility.

Sea state is another factor affecting light penetration. Rough seas reflect more light, thus reducing the underwater illumination. So if you’re choosing between two dive sites, opt for the one with calmer waters. The visibility will probably be better.

THE BIOLOGY OF VIZ

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