To See or Not To See, That is the Question
What is Visibility?
Visibility, quite simply, is the prevailing distance at which an object can be seen when looking horizontally underwater. Of course, there are factors that influence the visibility of an object underwater including its size, shape, and contrast against the background. And visibility is affected by light levels and depth so even really clear water will have poor visibility if the sky is cloudy and you're at 100 feet.
What is Good Visibility?
I asked this very question after my first dive. Having nothing to reference against, I thought it reasonable to ask the dive master if what we just experienced was considered good visibility. His answer was, "yes 80 feet is good, but diving in Belize can get better."
You've probably heard this before, but different colors disappear at different depths -- red, followed by orange, yellow, and then green. Last to go is blue, which if you're into underwater photography, is the overall color cast of most flash-free photos.
Determining How Far You Can See
Ask a dozen divers what the visibility is of a dive and you'll get a dozen different answers. The fact is that visual effects and illusions can influence a diver's perception. For example, the magnification effect resulting from looking through a mask makes nearby objects appear larger (and closer) than they actually are. And curiously, this effect reverses such that large objects in the distance are often thought to be farther away than they actually are again due to how are brains interpret the magnification.
Assessing visibility by looking down from the surface isn't foolproof either. If there's a thermocline near the surface, visibility will appear to be worse than it is at the depth you're diving. Looking at the surface from the bottom isn't all that more useful as the added contrast from the sun can make visibility twice as good as when looking horizontally.
What Affects Visibility?
The most common cause of reduced visibility are particles -- live or otherwise -- in the water. A sandy or muddy bottom can easily be kicked up and with small particles it can take a while before the sediment settles again. Shipwrecks and caves are particularly prone to fine silt.
Light is, obviously, a major factor in the visibility equation. The more that comes down the better the visibility. It follows then that when the sun is at its highest (midday), visibility will be at its greatest. What is often not considered is the impact of rough water. Waves and other turbulence will reflect light and cause it to scatter more than a smooth plane.
Weather and seasonal factors also come into play. Water run-off from mountains in the spring can increase the velocity of water flow in rivers. This in turn results in more sediment being picked up and carried into nearby bodies of water. From there, prevailing currents can carry the sediment to your dive site.
Diving in Low Visibility
You can't change water conditions, but you certainly can take measures to ensure a dive is enjoyable and safe:
- As with all dives, a buddy is critical to a safe dive in low visibility. As visibility decreases, the distance between you and your body should also decrease and in the worst conditions divers should consider physical contact. Dive lights are good to keep on hand as low light often accompanies low visibility.
- You'll need to switch from visual cues to tell where you are to compass navigation as familiar landmarks are hidden from view.
- Be more vigilant about entanglements as you're more likely to get caught by one in low visibility conditions.
- Be prepared to shift your focus -- rather than enjoying the vast expanse of a reef, focus on the details be decreasing the range between you and marine life.
Don't let the prospect of less than ideal conditions force you to cancel your dive. Even low visibility dives can be safe and interesting. All that's needed is preparation and know how to compensate for different levels of visibility. With a little planning you'll be on your way to getting more dive hours under your weight belt!