Once, a number of years ago when I was living in California, I was at the beach and encountered two former students of mine who were preparing to make a dive. I’m always glad to see that people I’ve trained remain active divers, and it’s enlightening to hear about their diving adventures. After a brief discussion and exchange of pleasantries, they were off on their dive, and my wife and I retired to a nearby dockside restaurant where, coincidentally, I could see the divers entering the water.
As we waited to order, I watched the intrepid pair enter the surf and begin to swim out to sea. The conditions were less than ideal, with a solid 10- to 15-knot wind blowing, and the visibility was only slightly above what I’d describe as pea soup. From the flotsam that drifted by offshore, I could also tell that the usually northerly longshore current was, for some reason, flowing south that day.
In retrospect, I should have suspected that something was wrong. But I was confident that my protégés would have no problems for two reasons. First, they were outstanding students — clearly several notches above the norm. Furthermore, the site they were diving was also the location of their final open-water training dives, and I knew they had dived there numerous times since.
Once they were out of sight, I turned my attention to my appetizer and didn’t give the divers much more thought — at least not until I saw a lifeguard boat being hurriedly launched and two barely visible forms bobbing well offshore, waving for assistance. Indeed, it was the dynamic duo who, unless the lifeguard reached them soon, were on their way to Japan.
After a safe recovery and landing — along with a strong admonishment from the lifeguard, who was previously sitting in a nice warm booth, confident he’d have little to do that day — they sheepishly made their way back to their van. As they came around the back of the vehicle to open the door — and saw me sitting on the bumper — I casually remarked, “Interesting dive, huh guys?” The expressions on their faces said all that was necessary.
“Let me get this straight,” I continued. “You are the same guys who were in my class last year and actually earned a certification, right?”
“Yes,” one answered in a barely discernible voice, while never making eye contact with me.
“What possibly possessed you two to even make a dive today in these weather conditions, and why in the world would you enter where you did with a south-flowing current?” I asked incredulously.
Their response was very simple. They were so familiar with the spot that they didn’t think it was necessary to determine the direction or strength of the current, and because the site is usually sheltered, they didn’t think that the poor weather would be an important factor.
Two lessons were learned that day. My students learned that no matter how familiar you are with a dive site, you never enter the water without thoroughly assessing the conditions present. On the other hand, I learned that I would have to give greater emphasis to this point in my classes.
Since then, I have developed a very thorough and, I believe, effective presentation to my classes on how to evaluate environmental conditions. This article is a synopsis. I hope it precludes any of you from emulating my former students.
Weather is the most important consideration in evaluating diving conditions because it impacts conditions both above and below the water. In fact, few other leisure activities are as much at the mercy of the weather as diving. But to properly evaluate weather conditions and their suitability for diving, you must first have at least a rudimentary knowledge of how the atmosphere behaves.
The first step in making an informed decision about weather is getting accurate information. That means getting a forecast from a newspaper, television, or radio. Perhaps the best source of information is the in-depth, marine-oriented reports broadcast by the National Weather Service. Anyone with a VHF-FM marine radio can find these reports on special channels, usually marked WX 1, 2, or 3. Some citizen band (CB) radios also have these channels. If you don’t have either a VHF or a CB, you can purchase a dedicated weather radio at your local electronics supply store. These inexpensive devices are designed to receive NWS weather broadcasts exclusively and even have a special activator switch that turns them on automatically when severe weather warnings are announced.
In the broadest sense, weather patterns form due to changes in atmospheric conditions caused by warm equatorial air masses interacting with cold polar air. The driving force of these air masses is the earth’s rotation, and uneven heating and cooling of the atmosphere by the sun. High-pressure areas are generally associated with sunny, calm weather. Low-pressure areas, however, bring clouds, rain, and wind. Although it may sound relatively simple and straightforward, predicting weather is enormously complex and — as anyone knows who watches TV weather reports regularly — not entirely accurate.
While that’s about all that most people need to know about weather, it’s not enough for divers, because weather patterns in coastal regions often differ from forecasts for inland areas. This is why a marine-oriented forecast is vital when you’re planning a dive along the coast.
For example, calm wind conditions in the early morning hours is a typical summertime weather pattern. But, as the sun heats land masses much faster than water, the land quickly warms and forms an upward convection current. This pulls cooler air in from the sea and creates a sea breeze. The wind usually intensifies until late afternoon, when it begins to dissipate. Nighttime often brings very little or no wind. Using this knowledge, you can normally assume that morning will bring the calmest wind conditions in coastal regions, with the wind building gradually throughout the day.
Still, this general pattern is not an absolute rule. Weather is influenced by many different factors and can behave differently from what’s been described. So regardless of your knowledge of how weather happens, there’s still no substitute for an in-depth forecast of the area where you’re planning your dive.
Another aspect of coastal weather makes even a marine weather report less than accurate at times. This normally occurs when offshore winds (blowing from shore) are forecast. An excellent example of this discrepancy between a forecast and reality occurs regularly along Florida’s east coast. Primarily in winter and spring, cold fronts blow into the region, causing strong westerly winds. When this happens, marine advisories are often posted because these winds can create dangerous sea conditions. But since it’s blowing from shore, the wind doesn’t have time to affect the seas close to shore. This results in calm seas within a mile or two of the beach, even though seas may be raging only a few miles farther east. Because many prime dive sites are close to shore, I’ve often experienced excellent diving conditions while small-craft advisories were in effect and the local TV weatherman was warning people not to venture out of the harbor. This points out the need to have not only accurate information, but a lot of local knowledge as well when assessing diving conditions.
When planning a dive in the ocean (or some very large inland lakes), currents are the primary environmental concern after weather. Often, when they’re severe, currents are caused by tidal exchange. So predicting current means being able to read a tide chart. In interpreting the charts, take special care on days when there’s a great variation between tides. When this occurs, vast amounts of water must flow between the high and low tide, creating local currents that are much stronger than on days with minimal tidal exchange. Under most circumstances, diving at high tide yields the best visibility because it brings with it clear, offshore water.
If you think current may be a factor in your dive plan, the ideal time to enter the water is around what’s termed a slack tide. This is the point when tidal currents cease in preparation for the change to the opposite direction (high to low or low to high). Logically, the greatest water flow takes place midway between the tidal exchange. For example, if a low tide occurs at 10 a.m. and a high at 10 p.m., the current is likely to be at its worst around 4 p.m. A slack high tide will occur sometime around 10 p.m., as the water reaches its crest. But tides can be influenced by a wide variety of factors, such as physical obstructions, bays, or even bottom topography. This makes determining the precise time of a slack tide in some locations very tricky. If you need to be dead accurate in an area where you’re unfamiliar, consult local watermen or use information from the Tidal Current Charts published by NOAA. This publication is available in any marine supply store and provides virtually worldwide coverage, as well as instructions for use.
In addition to tides, local currents can be influenced by major oceanic currents. Especially when these major currents interact with localized near-shore currents, predicting the behavior of water movement can be next to impossible. For example, the Gulf Stream flows from south to north along the U.S. east coast. So it’s logical to assume that all longshore currents on the eastern U.S. coastline flow unalterably in a northerly direction. But as anyone who has ever dived off that coast can tell you, that’s just not true. Indeed, at times the current does flow north. But sometimes it flows south, and sometimes there is no current at all near shore.
This doesn’t mean the Gulf Stream has ceased its flow or changed direction. These variations in local currents occur because major ocean currents aren’t stable; they meander and undulate like snakes slithering across the ground. Sometimes they move shoreward, sometimes seaward. Under the right circumstances, counter-currents called “eddies” spin off and cause localized currents in directions opposite the oceanic current. Just as with tides, local physical features have an impact as well. In the final analysis, understanding and predicting local currents takes extensive experience. If you lack that experience, it’s often best to leave this aspect of dive planning to professionals. This is only one of the many advantages of diving with organized groups such as those sponsored by dive clubs or dive centers. Learning under the tutelage of more experienced divers is both safer and less stressful than going it alone.
If you insist on planning your own dive, make sure you assess the sea conditions from the best vantage point — the highest spot on the shore or boat. In determining the direction and strength of the current, watch for either boats at anchor or flotsam on the water’s surface. Make sure it’s the current, not the wind, moving the objects in the water. Strong wind can push floating objects — even boats — against a mild current. Although uncommon, also be aware that a subsurface current might flow in a different direction than the surface current (sometimes by as much as 90 degrees). This can occur in open, deep water — such as you might encounter while diving from a boat.
When you must dive in a current, remember it’s the ocean that controls things, not you. For instance, you can only swim against the slightest current (usually less than 2 knots), and even then for only a short distance. So instead of fighting the ocean, use it. Plan your dive so the current is an aid, not an enemy. Consider drift diving if the current is especially strong. Or, if you must anchor-dive or dive from shore, plan to end your dive up-current of your entry point. This lets you use the current to bring you home. It’s at the end of the dive when you’ll be most tired. Equally important, never assume that because the current is flowing one way when you enter the water, it can’t change direction. During your dive pay close attention to anything that might indicate a change in the direction or speed of the current, such as aquatic weeds, soft corals, plankton, or other particles suspended in the water column.
If you nonetheless get caught in a strong current, don’t try to fight it. In any fight with the ocean, you will lose. Inflate your BCD, drift with the current, and signal for assistance. Hopefully, if you’re diving from a boat, it will have a drift line trailing from its stern for just this situation. Although it may be embarrassing, you can always get back to your exit point somehow. But exhausting yourself by fruitlessly battling a current could be deadly.
How’s the Vis?
The reason we dive is to see what’s down there, so visibility is an extremely important factor in evaluating a dive site. How calm the sea is or how little current you have to contend with is irrelevant if you can’t see your hand in front of your face once you’re underwater.
There are a myriad of factors that influence visibility, including weather, seasonal variation, bottom composition, water motion, and even the time of day. All of these affect either how much particulate matter is in the water (turbidity) or the amount of light that can penetrate underwater (absorption).
Weather and seasonal variation have a significant impact on visibility. Near shore, rain causes runoff, which can greatly increase the number of light-robbing particles in the water. Both near-shore and offshore warm temperatures and increased sunlight during summer can produce conditions which cause explosive plankton blooms. (This is one reason that in cold-water regions like the Pacific Northwest visibility tends to be better in winter than summer.) In fresh water particularly, changes in subsurface thermal stratification caused by seasonal temperature variation greatly affect visibility. For this reason, predicting visibility in fresh water often requires special knowledge and a great deal of experience.
The most obvious factors affecting visibility are the composition of the bottom (also termed substrate) and water movement. Severe surface chop or wave action decreases the amount of light that can penetrate the water. Bottom composition, on the other hand, affects turbidity. As a rule, the finer the substrate, the easier it can be disturbed and become suspended in the water. The easier the substrate can be disrupted, the more likely water motion such as waves, current, or surge will affect visibility.
Bottom compositions such as mud and silt are much more prone to the effects of water motion. Rock, clay, course sand, or coral are less affected. Aquatic weeds also help to keep the bottom more stable and less susceptible to water motion. This means that when planning a dive you should select a site with consolidated bottom materials — rock, course sand, coral — whenever possible. If you’re not sure what kind of substrate to expect, consult a nautical chart. In addition to telling you what kind of bottom topography is present, they also indicate the bottom composition. If you must dive on a muddy or silty bottom, maintain good buoyancy control and make sure you keep your fins away from the bottom. (Anti-silting procedures are important skills in cavern diving, and mastering these techniques is an excellent reason to take a Cavern Diving specialty course, even if you don’t intend to do a lot of diving in caverns.)
Although rarely considered except by underwater photographers, the time of day you make a dive can also be a significant factor in the quality of underwater visibility. A physical property of light is that when it strikes the surface of the water at an angle of less than 48 degrees, most of it is deflected. The higher the angle, the more light can penetrate. Taking this to a practical level, the highest light penetration, and the best visibility, normally occurs when the sun is at an optimal angle — between 10 a.m. and 2 p.m.
The Critter Factor
Marine life presents the most unusual challenge to evaluating dive conditions, because its effect can be both positive and negative. On one hand, few things are more frightening to divers than their unrealistic perceptions of the creatures that live underwater. The “Jaws mentality” seems to hold that if something can eat you, it will — or at least try. On the other hand, the main reason most of us dive is to experience these unique critters that foster our fear. How do we deal with this paradox? By striking a balance between reason and fantasy. Certainly, there’s always some risk from interacting with wild animals in their own environment. But the potential risk must be put into perspective. After all, injuries from marine life are almost always a result of defensive action on the part of the animal rather than aggression.
Most enlightened divers today believe that we should concentrate not on protecting ourselves from marine residents, but on protecting them from us. It seems that education and fear are inversely proportional; the more you gain of one, the more you lose of the other. So the answer lies in learning more about what’s down there. To do this, consider taking a specialty course in the marine environment, fish identification such as the ones sponsored by the Florida-based R.E.E.F. Foundation, or an underwater naturalist program. If you really get hooked (forgive the pun), you might think about a formal marine science course at a local college. But even if such courses aren’t available or won’t fit into your schedule, there are still other alternatives. Today a number of excellent texts and field guides exist on virtually all aspects of the marine environment. Whether you’re interested in coral reefs or diving more frigid but no less spectacular waters, information abounds. All you need is a desire to learn.
Learning how to accurately assess diving conditions takes time. While the basics are pretty simple, the intricacies of how environmental conditions interact can be confusing and even misleading. That’s why it’s best to gain experience under a more seasoned buddy or through a continuing education course. But even experienced and highly trained divers aren’t immune to mistakes, particularly when they dive an unfamiliar site. Whether you’re a novice right out of a certification course or an experienced instructor, seek assistance when you dive a new area for the first time. Many training organizations have special programs just for this purpose. Consult a dive center in the area you plan to dive for advice. Remember, it’s not the environment that causes problems, it’s the decisions you make in evaluating the environment.
A Quick Reference to Evaluating Diving Conditions
Before you arrive at the dive site:
• Get accurate weather information. Call off the dive immediately if severe weather is forecast. Remember that it takes time for weather to influence sea conditions and for sea conditions to recover once severe weather dissipates. Make sure the forecast you use applies to the area you’ll be diving.
• Consult tide charts and try to schedule your dive to take place at a slack high tide. When diving on a commercial charter, tides may be less of a factor, and the boat will probably operate on a fixed schedule regardless of the tide. Ask the captain or crew about any potential effect of tides.
Once on site:
• Carefully observe the sea state and evidence of current. Take several minutes to do a thorough assessment. Sometimes the effects of currents are subtle, and an oversight or bad decision can mean a bad dive or worse. Again, on a commercial charter boat, the crew will probably discuss the effect of current during the briefing.
• Evaluate the visibility. How does it appear? Remember that vertical visibility is normally better than horizontal, so don’t overestimate. Determine if there are any conditions present that might cause a change in visibility, such as building wind, changing tides, or threatening weather. Is the visibility sufficient to fulfill your dive objective and plan? In poor or deteriorating visibility, make sure you review what to do if buddies separate. Also consider what impact the bottom composition might have on visibility.
• What kind of marine life will you encounter? This can depend not only on locale but the season, too. For example, some fish species are migratory, and in many locations some forms of jellyfish are seen primarily at certain times of the year. You should also have a field guide covering the local marine life when possible.