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 DUEX", 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. Just click on "PART DEUX"
below.
Any comments, complaints, gripes,
grumbles, griefs, laments or
accusations can be addressed to our webslave ......
Wally Barnes