SCUBA MIAMI

WHY DO WE NEED TO KNOW TIDES??? * The following article appeared in SOURCES, the NAUI Journal of Underwater Education. Reprinted here with permission from NAUI. Our thanks to their staff for permitting us to share this important information with our readers.* PART 1 INTRODUCTION Tides is that fluctuation ... the ups and downs ... in the sea level, day after day, hour after hour. What are the forces behind such movements??? Why does it rises higher on certain days??? How does the tidal current forms??? How does it affect the dive plans??? Where do I get day-to-day information on the tides??? As divers, we need to include the expected tidal current estimate in our plans and prepare for its effect during our time underwater. We need to know how strong a tidal current to expect and at what time it is at its maximum. Also, at what time does it becomes slack (close to zero) and how fast would the strength change. This article, divided in three parts, will try to explain ... in a very overly simplistic way ...... some of the impact the tides, hence, the tidal currents, have upon scuba divers. It is NOT INTENDED to be universal, not even locally, but general in nature and in NO WAY should be construed to be representative of any particular site. Users are advised to seek local information regarding differences between the strength and timing of tides and tidal current maxima and slack waters. Let's begin with the forces that make the level of the sea rise and fall. We'll then relate this change of level with the currents, its strength and timing. Every celestial body exerts, and at the same time is exerted by the Earth, an attracting gravitational pull and an equal, but opposite, centrifugal outward force. For simplicity, we'll consider the sun and the moon as the only two bodies exerting gravitational force on the Earth, since the effect from other planets or stars is negligible for our purposes. THE MOON One effect the moon has over the oceans is to change the level. At the Earth region closest to the moon, the sea level rises because it is under maximum gravitational pulling force. On the diametrically opposite side of the Earth, at the region of maximum centrifugal push, the level would also rise. To raise the level of the sea, water must move unto that point from an adjacent area. To lower the level, water moves out. This horizontal movement is called a TIDAL CURRENT and is of great importance to divers because it can make a difference of whether you can return safely to your boat or beach or NOT. We'll discuss these effects on divers shortly. When water is attracted into an area and raises the level, it is moved out from somewhere else, thus lowering its level. The two regions ... the closest to the moon and the farthest ... have higher water levels and become HIGH TIDES. The two regions to the sides (at right angles) have lost water and have lower levels, thus becoming LOW TIDES. With the Earth making a complete revolution each 24 hours, this gives us two high tides and two low tides a day in most open ocean areas. We have a high tide, then approximately six hours later, a low tide, six hours later another high tide and finally, six hour later another low tide ... and ready for the next day of ups and downs. We can complicate matters by adding that while the Earth is turning, the moon advances the equivalent of one hour. So the cycle is really one of 25 hours. Every day that passes delays the next high or low tide by approximately one hour. So the tide that was at 11:30 pm tonight will be at 12:30 am next day ... explaining why some days come short of one tide. THE SUN The sun would exert an even stronger centrifugal / gravitational force over the seas because of its larger size, but its impact is decreased by its enormous distance compared by the proximity of the moon. Similar to the moon, the regions closest and farthest from the sun will raise the water level and the two regions at right angles will lower their levels ...... but not as high or low as the moon's rise and fall due to the sun's weaker effect. Also the cycle is now 24 hours not 25 as in the moon's case. In summary, we have two forces affecting the seas (imagine if we added all other planets and stars!!!) which are similar, but differ in strength and timing. BOTH SUN AND MOON Now we really get complicated !!! What happens when the sun and the moon are aligned and the regions of maximum attraction coincide??? You've got it!!! ... a very high tide on one side of the Earth and another one on the diametrically opposite region. This rise in the level of the oceans means a significant lowering of the water levels in the regions at right angles. So when this in-phase of the moon and sun occurrs, which happens whenever we have a full moon or a new moon ... we get very high tides and its sidekicks ... very low tides. This is referred to as SPRING TIDES. Their relation to divers is critical because the very large amount of water moving from one place to another means currents are going to be very strong (remember ... you only have six hours to rise or fall from one extreme level to another). What about when their alignment is out of phase??? When the moon pulls water up towards it while the sun is pulling too at a right angle ... the moon gets the high tide because its closer distance makes its effect stronger. But since there is a lesser force (sun) attracting at the regions losing water, the level at the moon's side doesn't rise too much ...... while the water level at the sun's side doesn't lose too much either, so we have a not-so-high tide and a not-so-low tide. This we call NEAP TIDES and it happens whenever we have the moon in first or last quarter. Their relation to divers is important because here the tidal current is relatively weak and, even at maximum strength, it usually isn't so strong. If divers could learn to identify which moon phase when planning their dive, they can time it to avoid strong currents. SUMMARY So far we have come up with several conclussions : ONE ... The moon and the sun are the causes of the tide changes. These changes produce the tidal currents. The phase of the moon is the most important factor in the strength of the tidal current. TWO ... During full moon and new moon, the tides (SPRING) will have very high high tides and very low low tides. Because of this large difference the maximum tidal current will be very strong. THREE ... During first and last quarter moon, the tides (NEAP) will have not-so-high high tides and not-so-low low tides. Consequently, the small difference in levels will produce a relatively weak maximum current. In our next tidal discussion, "PART DEUX", we explain how to tell the timing of the maximum and minimum tidal movement and how our dive plan can be changed to fit the best currents ...... sorry, we haven't figured out how to change the tidal currents to fit our dive plans. We also discuss how to use the tide tables that appear in your daily newspaper to figure out the secrets of the tidal currents.
*** The following article appeared in SOURCES, the NAUI Journal of Underwater Education. Reprinted here with permission from NAUI. Our thanks to their staff for permitting us to share this important information with our readers.* PART 2 This article is the second of three parts on the diving impact of tides and tidal currents. Please refer to part one before reading this section as we are expanding concepts explained there. We repeat that this article is NOT INTENDED to be universal, but general in nature and in NO WAY should be construed to be representative of any particular site. Users are advised to seek local information regarding differences between the strength and timing of tides and tidal current maxima and slack waters. ANGLE AND DISTANCE OF THE SUN AND MOON Not to get too boring with technicalities, but let's remember that the sun and moon are not always in the same celestial plane. When they are, we get an ECLIPSE and the points of maximum centrifugal and centripetal force coincide. Then, we really are talking about extremely high HIGH TIDES and extreme low LOW TIDES ... and its consequential extremely strong TIDAL CURRENTS. Of course, eclipses occur on new or full moons ... SPRING TIDES ... only. COULD WE COMPLICATE MATTERS MORE ??? Of course !!! We could indicate that the moon, at its closest distance (perigee) from Earth, the stronger its influence. By the same token, the farther the distance, the lesser the effect over the tides. With a decreased moon effect, the lesser the change of tidal height. Remember, the higher (or lower) the tide, the stronger (or weaker) the tidal current ...... and this is what divers must understand to avoid a face to face encounter with a strong tidal current. We could also add meteorological effects on currents, like strong wind blowing or water accumulating against land masses and rushing back or rivers and estuaries discharging unusual amounts of water causing currents. But things are stuffy enough the way they are to add those irregular and highly unpredictable events. Let's stick to tides and its currents. This was the case of our last "Wreckers Delight" Liveaboard trip to the Florida Keys (see window in main menu) ... where we visited the Duanne, the Bibb and the Eagle Wrecks (all deep wrecks) in one weekend. We just had an eclipse the day before departure, under a gorgeous harvest full moon which was just in its perigee the day before ... and believe it, all those factors made the current streak by really howling!!! HOW TO USE TIDE TABLES Divers don't need a PhD to tell them that currents are critical to a person jumping from a boat and swimming underwater several hundred feet away. A diver's average speed is about 100 ft/minute, which is around one knot or one mile per hour (give or take). A diver can usually cope with currents less than 60 ft/minute ... anything above that would cause over-stress and exertion (depending on stamina, equipment and physical conditions). Tidal currents can vary from zero to well above 200 ft/min ... that's why dive planning MUST include the tidal current factor in it. Let's get into the nitty gritty of the article. When a region in the ocean is at high tide, we assume water is no longer coming in (otherwise the level would continue to rise). That means the incoming current is ZERO. The same logic follows for low tide. When at low tide time, no water is outgoing or the level would continue to fall. The TIDAL CURRENT AT THE TIME OF HIGH OR LOW TIDE IS NEAR ZERO, or at least so weak (less than 20ft/min), we call it SLACK water. Returning to our original point of high tide, ... now the water will begin to flow toward other sites which have rising levels and a tidal current starts. Slowly at first ... then stronger and stronger ...... until a maximum strength is reached at about halfway to the next (low) tide time ...... roughly three hours (since we have six hours between high and low tide). From that maximum at the midway point, it begins to weaken until it reaches a ZERO again at the next tide time. In the tide tables, the information you need is the time of the high and / or low tide. We will consider these times as the slack (near zero) current. We will then estimate that halfway between the slack times, the current will be at its maximum strength. EXAMPLE :** With a high tide at 13:03 (1:03pm) and a low tide at 19:21 (7:21pm) ... we start with slack current (near zero) at 1:00pm. Then, there will be a weak out-going movement getting progressively stronger until 4:10pm, when it reaches its maximum speed, then weakening until it is back to near ZERO at 7:20pm (time of low tide). It will now reverse the flow and a weak incoming current will start rising the water level becoming stronger and reaching maximum strength halfway to the next high tide ... at which time it will zero again and start the cycle all over.	We now know when the currents are going to be stronger or weaker, but how strong??? The answer is in the moon phase. At SPRING TIDE ... new or full moon ...... that maximum current is bound to be very strong and should be avoided by divers ... unless, of course, you are doing a drift dive. If you are planning a reef dive that day, make sure you're in the water very close to 1:00pm with the SLACK current and out of the water before 2:30pm when the increase in current speed becomes sudden and dangerous. On the other hand, if that dive was planned during a NEAP tide ... first or last quarter moon ... the dive could be carried out even at 4:00pm since its strength is NOT likely to be extreme. AGAIN, THESE ARE GENERAL CONCEPTS OF TIDAL CHANGES AND DO NOT APPLY UNIVERSALLY TO ALL PLACES AND AT ALL TIMES. CONSULT YOUR LOCAL DIVING AUTHORITIES WHEN IN DOUBT OR TO LEARN LOCAL FACTORS INFLUENCING THESE CONCEPTS. Check your local tide tables, notice the height of the high tide and of the low tide; open ocean differences of three (3) ft or more usually cause currents well above 60 ft/min. In the above example, dive near the time of high or low tide ... when the current is SLACK (high tide, because it brings clean water from the deep sea is usually clearer than low tide which normally brings dirty water from nearshore). During NEAP tides, the difference from high tide to low tide is small (less than 1.5 ft) and the water rush is NOT too strong, so your timing is not really critical. Of course, if your swimming skills are not fully developed, don't take chances, dive when the currents are slack whether it is SPRING or NEAP tide. Where can you get tide information??? That's the easiest one. Check your local newspaper, your TV weather reports or even radio. Even yesterday's data is good, just add one hour to that time for as good approximation. To quickly summarize what we have discussed here : ONE...Tidal currents are SLACK near low or high tide time. That's four times a day. Maximum current speed is halfway between tide times. TWO ...Strong maxima during SPRING TIDES ( full or new moon). Weak maxima when NEAP tides (first or last quarter moons). THREE ...Your local newspaper carry the tide time information or local TV weather reports. Now that we know how tides relate to current, its effect on divers and the times to avoid strong tidal currents ... and just as important ... how and where to get this information, we can make it an integral part of our dive plan. It is only to your own benefit and safety. Many books emphasize this importance. Now you know how to use it!!! On the third part of this article, "PART TROIS", we discuss deviations from these generalities and quantifying some of the strong or weak currents. Then a knowledgeable diver can better prepare to cope with strong currents or weak ones as the case might be.

******** The following article appeared in SOURCES, the NAUI Journal of Underwater Education. Reprinted here with permission from NAUI. Our thanks to their staff for permitting us to share this important informationwith our readers.****** PART 3 This article is the third of three parts on the diving impact of tides and tidal currents. Please refer to Part One and Deux before reading this section as we are expanding concepts explained previously. We repeat that this article is NOT INTENDED to be universal, but general in nature and in NO WAY should be construed to be representative of any particular site. Users are advised to seek local information regarding differences between the strength and timing of tides and tidal current maxima and slack waters. In this article we expand on other astronomical phenomena that influence the height of the tide and, therefore, the strength of the maximum current. Please refer to the previous two articles on the subject. DISTANCE OF THE MOON The attraction force between our planet and the moon varies with their distance apart; the farther apart, the less attraction and vice versa. When the moon is on its perigee, it is on its closest point to the Earth and the tidal currents' maximum strength will be intensified. Upon reaching its apogee, or farthest distance from our planet, the moon's effect will be decreased and the maximum current strength will be reduced. The effect of the moon's apogee and perigee on the tidal current is irrelevant to whether we have spring or neap tides. Let's quantify these forces in an oversimplified way. Please remember these numbers do not, in any way, reflect a real current strength. They are used only to help establish a reference point. Local currents may vary significantly. Check your local diving authorities for more appropiate information. If we assume that the average strength of a tidal current is around 50 ft/min (a normal diver moves at 100 ft/min and this is only for a few minutes), we would expect that: Max current Perigee Apogee Spring Neap = Total 50 ft/min +10 +25 = 85 ft/min 50 " -10 +25 = 65 " 50 " +10 = 60 " 50 " -10 = 40 " 50 " -25 = 25 " 50 " -10 -25 = 15 " The speeds range from a maximum current so slow that any diver could cope, or so strong that none could. Keep in mind we only refer here to the maximum current occurring halfway between tides (usually about three hours after a tide or before the next one). The minimum or slack time occurs close to the time of the tide and could go as slow as 0 ft/min (no current). Other currents, like global or wind driven currents are not included in any of these calculations. They have to be determined by the diver before s/he jumps in the water. But, because these currents do not normally reach the strength of the tidal flow, they usually are not accounted for in a dive plan. ANGLE OF THE MOON** Most of the information offered here works well when the moon is at or near the Earth's equator. In other words, events ... highs and lows ... are symetrical. While this simplifies explanations, we still have to deal with the case of the moon traveling to the north and south of our equator, consequently displacing the high tide sea level bulges and the low tide depressions.	The moon travels once in a cycle as far as 28.5 degrees north and south of its plane of rotation. In the USA, that would be about the latitude of Cape Canaveral, Tampa and Corpus Christi. An observer at these locations would see a maximum high tide with an overhead moon, meaning a stronger maximum tidal current for whatever moon phase (spring or neap). But 12 hours later, s/he would then find a not-so-high tide because the high tide bulges are not symetrical ...... producing a weaker maximum tidal current. In Figure 1, we see the diver under the moon's zenith in the northern hemisphere with a very high spring tide. Half a day later, s/he is at a not-so-high tide on the other side of the planet. Of course, now the change of tide height is minimum, therefore, the maximum tidal current is weak. So on days when the moon is at or approaching its northernmost or southernmost points in its orbit, it would be smart to dive when the tidal change is minimal and maximum tidal currents are weak. So, rather than a morning dive with a strong current, plan on an afternoon dip with little or no current. You'll enjoy it more and be safer. Let's get our conclusions together here now : ONE ... Moon perigees intensify the "normal" tidal current maximum current. Moon apogees decrease the "normal" tidal current maximum current. We can find information on the moon's orbit on any nautical almanac or in the newspaper clipping on the tides. TWO ... The tidal current speed is the resulting combination of forces affecting the main water in (or out) flow including apogee, perigee, neap or spring tide. THREE ...The moon at its northernmost or southernmost position will produce one very high tide and one very low tide, then one not-so-high tide followed by a not-so-low tide ... so there will be one strong maximum tidal current and one weak maximum tidal current. This is regardless of whether it is a spring or neap tide. Divers must be aware of the forces the moon exert upon our oceans. Very few events will scare a diver more than being pulled away from the return point by an uncontrollable force. From trying to counter an overwhelming current comes fatigue and from fatigue comes despair, from despair, panic, and from hours waiting for a rescue, a very chilled and disgusted diver (at best!!) LOCAL FACTORS INFLUENCING THESE CONCEPTS. Check your local tide tables, notice the height of the high tide and of the low tide; open ocean differences of three (3) ft or more usually cause currents well above 60 ft/min. In the above example, dive near the time of high or low tide ... when the current is SLACK (high tide, because it brings clean water from the deep sea is usually clearer than low tide which normally brings dirty water from nearshore). During NEAP tides, the difference from high to low is so small (less than 1.5 ft) and the water rush is not too strong, so your timing is not as critical. Of course, if your swimming skills are not fully developed, don't take any chances, dive when the currents are slack whether it is spring or neap tide. Now that we know how tides relate to currents, its effect on divers and the times to avoid strong tidal currents ...... and just as important, how and where to get the information ..... we can make it an integral part of our dive plan. It is only to our own benfit and safety. Many books stress the importance, but fail to explain how you make your calculations to know when to jump in. Any comments, complaints, gripes, grumbles, laments or accusations can be addressed to our webslave ...... Wally Barnes