As a diver you have probably heard a lot about tides and currents and you know that they probably influence your dive and perhaps even what time the boat leaves to the dive site. But what are they really, what causes them and how often do they change. Here is a quick crash course on Tides and currents that you can use the next time you want to impress that cute dive instructor!
Tides and currents
What’s the difference between tides and currents?
Put very simply, tides can be described as slow, long waves moving from the open ocean to the shore. They are created because of the forces or ‘pull’ that the sun and the moon have on the earth (and through this, on the water masses on earth). When the highest part of the wave reaches the shore we call it high tide. When the lowest part of the wave comes to shore we call it low tide.
When the tide moves toward (approaching high tide) or away from (approaching low tide) the shore it also causes a horizontal flow of water. This is called a tidal current. Usually the tidal current is strongest closer to the time of the high and low tides. Currents are usually weaker when the water has reached either the high or the low tide, before it ‘turns’ to move in the opposite direction. This is called the slack tide.
What causes the tides?
As I mentioned earlier: Tides are caused by the gravitational pull of the sun and the moon on the water. Although the sun is bigger and might have a larger gravitational pull, the moon is closer to earth and thus have a larger effect on the movement of the oceans. The water on the side of the earth that is facing the moon gets pulled towards the moon, creating a bulge of water on that side. A phenomenon called inertia (the tendency for a moving object to keep moving in that direction) creates a water bulge on the opposite side of the earth. The area where the bulge is experiences the high tide while the areas where there are no bulge experiences low tide.
As the earth moves around the sun, and the moon around the earth, the position of the earth, sun and moon in relation to each other changes constantly. These changes influence the location of the water bulges. As a result of that there is a change in the heights of the tides and consequently a change in the strength of the tidal currents. These changes occur on a daily, and even hourly basis.
Tides caused by the gravitational pull of the sun are called Solar tides while the tides caused by the moon are called Lunar tides. Lunar tides are about twice as big as Solar tides. Generally when we speak about tides we are referring to the combination of Lunar and Solar tides.
When the sun, moon and earth are in a straight line (during full or new moon) the sun and moon pull together, creating extra high, high tides and extra low, low tides. This is called a spring tide. Because there is a large change in the height of the tides in a small time, the water moves fast, creating stronger tidal currents.
About a week later the sun and moon are at a right angle to each other, essentially each partially cancel out the pull that the other has on the earth’s water which leads to moderate tides. This is called neap tide. Smaller changes in the height of the tides means less water moves during the specific time and thus there are less strong tidal currents.
Both spring tides and neap tides occur twice in each lunar month.
The distance between the earth and the moon, and between the earth and the sun influence the tides, as there is a greater pull when either the sun or moon is closer to the earth. Once a month the moon moves close enough to the earth to create above-average changes in the tides. Two weeks later the moon is further away from the earth, leading to below-average changes in the tides.
Once a year (on January 2nd) the earth is closer to the sun, leading to exaggerated tides, while on July 2nd the earth is the furthest away from the sun, leading to smaller changes in the tides.
Frequency of tides – The Lunar day
We are all familiar with the solar day, the 24 hours it takes for the earth to rotate so that a specific place on the earth moves from an exact point under the sun to the same point under the sun.
A Lunar day is when a specific place on the earth moves from an exact point under the moon to the same point under the moon. The Lunar day is 24 hours and 50 minutes long.
During this Lunar day the earth revolves through two tidal bulges and thus most coastal areas experience two high tides and two low tides every 24 hours and 50 minutes. Thus high tides are around 12 hours and 25 minutes apart while it takes about 6 hours and 12.5 minutes to change from high tide to low tide and the same from low to high tide.
There are a few other things that influence the tides besides the sun and the moon. For example the shoreline, the shape of bays and inlets, and local wind and weather patterns.
In conclusion, if you are looking at doing a drift dive speciality, book your next dive trip during the full or new moon. If you are looking at very relaxed and easy diving, plan your trip for the phases in between.
*A version of this article first appeared at www.scubadiverlife.com