Tides are the rhythmic variations in sea level that we observe day to day. These changes in sea level are caused by the gravitational attraction of the sun and the moon on the Earth. Many other factors influence the nature and intensity of the tides, including the shape of the ocean basin and the Coriolis force. Together, these factors create high and low tides. Depending on the position of the Earth with respect to the moon and the sun, differences in the height of sea level during the high and low tides may be great or small. Many organisms, especially the California grunion, time their reproductive cycle to coincide with the tides.
Tides are the periodic variations in sea level caused by the gravitational attraction of the sun and the moon on the Earth. The force of this attraction creates very predictable rises and falls in sea level as the Earth rotates on its axis. When sea level is at its greatest height, the tide is said to be high. When sea level is at its lowest extent, the tide is said to be low.
High tides bring water far up on the shore. When wave action is high, these high tides may damage homes and undercut sea-side cliffs. On the other hand, low tides expose great expanses of the beach. During these times, marine organisms living on the shore may experience long periods of time (up to 12 hours) exposed to the air.
The gravitational attraction of the sun and the moon on the Earth depends on the distance of these bodies from the Earth. The closer two planetary bodies, the greater their gravitational attraction. Thus, the moon has a greater effect on our tides than the sun even though the sun has a greater mass (which also influences the degree of attraction). In general, the moon has about twice the effect on our tides than the sun.
Let's look more closely at what these gravitational forces do to the ocean. Consider first the simple effect of the moon on the Earth. Because of the gravitational attraction of the moon, the ocean on the side of the Earth facing the moon bulges outwards. This bulge is represented by Point A in the figure below. The Earth itself (Point B) also is attracted towards the moon, but because the center of mass of the Earth is slightly further away from the moon than the ocean on the side facing the moon, the gravitational attraction is not quite as strong. So the ocean on the side facing the moon is pulled the farthest and the Earth itself not quite as far.
This distinction is very important when we consider the origin of the bulge on the side opposite the moon. This bulge is always present. Even if the sun and moon are on the same side, there is a bulge of water on the opposite side. Why?
As shown in the figure, the attraction of the gravitational forces on Point C is not as strong as the forces on A and B. Thus, this water gets left behind, so to speak. The Earth is pulled away from the water on this side and a bulge appears.
You can convince yourself of this by putting two springs (or any material with elastic properties) between three balls in a row. If you pull on the first ball (represented by Point A), the second ball (represented by Point B) moves forward, but not as far as the first ball. The third ball (represented by Point C) also moves, but even less than the first two balls.
The sun has the exact same effect on the tides, although its attractive force is about half the attractive force of the moon. The bulge created by the sun is sometimes called the solar bulge while the bulge created by the moon is sometimes called the lunar bulge. Together, they add or subtract to create the tides.
Well, we've made two bulges. How does that translate into tides? Recall that the Earth rotates around its axis. (If you can't point in which direction the Earth rotates, then you should go to bed without dessert. Think about it. In which direction does the sun rise. Use your finger to point and call it out loud. That is the direction that the Earth is rotating towards.) As the Earth rotates on its axis, ocean basins are alternately carried underneath and away from these bulges. Where the forces that cause the bulges exist, sea level rises, causing the high tide. Where the gravitational forces don't exist, we have less water and a lower sea level, causing low tides. Because the Earth rotates beneath two bulges and two "troughs" (not to be confused with wave troughs), we have, in general, two high tides and two low tides every day.
In general, a complete tidal cycle takes 24 hours and 50 minutes. This is the time it takes for the Earth to rotate on its axis back to its original position with respect to the moon, the primary tide-causing force. Because it takes the moon about 27.3 days to complete one orbit around the Earth, the moon moves a little bit further around the Earth each day. Thus, the time of the tides advances about 50 minutes each day.
You can use two pieces of round fruit to convince yourself of this (oranges or grapefruits work fine, bananas don't). Mark a spot or insert a toothpick into a piece of fruit we will call Earth. Start the Earth fruit rotating on its axis and move the moon fruit forward about 1/27 of a complete orbit. You will see that the toothpick must travel that 1/27th further to catch up with the moon. Repeat several times until this concept is clear to you. (Then make yourself a fresh-squeezed fruit drink with the Earth and the moon!)
On top of the movement of the moon around the Earth, the Earth is also orbiting around the sun. In fact, the Earth moves "forward" around the sun slightly less than 1/12 of a complete orbit during one revolution of the moon. This extra 1/12 distance means that it actually takes the moon some extra time to line up again with the Earth and sun. Thus, for the purposes of the tides, which depend on the alignment of the Earth-moon-sun system, the monthly tidal cycle takes 29.5 days to complete. You will need three pieces of fruit and a friend (as an extra pair of hands) to convince yourself of these motions.
All of these motions, the Earth's rotation on its axis, the moon's orbit around the Earth and the Earth's orbit around the sun influence the height of sea level and thus, the height of the high and low tides at any time during the 29.5-day period. \
Patterns in the Tides
Recall that the shape of the ocean basin and the Coriolis force also have an effect on the tides. We won't discuss the Coriolis force and its effect in creating what are known as amphidromic points, but I will mention that this knowledge is necessary if you truly want to understand tides.
However, the Coriolis force and the shape of ocean basins create some fundamental differences in the tidal pattern discussed above. For example, the shape of the Gulf of Mexico creates a tidal pattern in which we have only one high tide and one low tide every day. This tidal pattern is called a diurnal tide.
Along the east coast of the United States, the two daily high tides and low tides are about equal in height. We wouldn't necessarily expect this because we wouldn't expect the two bulges to be the same. However, these other factors influence the tides and create the semidiurnal tide we observe in places like Boston and New York.
In California and the west coast of the United States, the two daily high tides and low tides are unequal in height. We call this pattern a mixed tide. Mixed tides can be broken down further on the basis of their height. The highest of the high tide in a day is called the high high tide. The lowest of the low tide in a day is called the low low tide. The lowest of the high tide is called a low high tide and the highest of the low tide is called a high low tide. Got that?
There is one more important pattern to the tides that we need to be aware of. As shown in the figure above, the position of the moon varies throughout the tidal cycle (every 29.5 days). The position of the moon with respect to the Earth is what gives us the different phases of the moon. When the moon is directly overhead at night, we have a full moon. When the moon is directly behind us at night, we have the new moon (or no moon). When the moon is at a 90 degree angle to the Earth with respect to the sun, then we have the first and third quarters of the moon.
We might expect that when the sun and the moon are on the same side or directly opposite sides of the Earth (during the full and new moon), their influence on the Earth will be the greatest because they will act together to cause the tides. This is exactly the case, as shown in the figure below.
During the full and new moon, the "size" of the bulges are greater; the lunar and solar bulges add together, creating a bigger difference between the high and low tides. When this happens, the tidal range, the difference between the highest and lowest tides in a day, is greatest. The period of the month when tidal ranges are at their greatest are called the spring tides. The tides appear to "spring up" between their high and low periods. (Now don't get confused, spring tides have nothing to do with the season.)
When the moon-Earth-sun system is at a 90 degree angle, the lunar bulge and the solar bulge tend to cancel each other out. During these times, the tidal range is minimal. The differences between the height of the high tide and the height of the low tide are small. This period of the month is called the neap tides.
Now, think about the effect of these planetary movements on the tides. In one tidal cycle (29.5 days), what are the phases of the moon? We have the new moon, the first quarter, the full moon, the third quarter and back to the new moon. So, how many spring tides and how many neap tides do we have in a month? If you answered 2 each, you can have your dessert now. Yes, we have two spring tide periods and two neap tide periods, each of which last a little over 7 days each (one-quarter of 29.5, technically).
Reading a Tide Chart
A tide chart is a graphical representation of the tidal cycle. Because the laws of planetary motion, the forces of gravitational attraction and the other tide-causing forces are so well known, we can predict the height of sea level with pretty good accuracy. These predictions are published by various governmental and private agencies into little booklets, called tide tables, or calendars, called tide charts.
The tide chart shown below represents the tidal pattern for one day of tides in Moss Landing, California. Time appears along the X-axis (the horizontal axis) and the height of the tide appears along the Y-axis (the vertical axis). The hills and valleys shown on the chart represent the height of the tide at a given time. Studying this chart carefully, we can see the presence of two "hills" or high tides and two "valleys" or low tides. As mentioned above, we name the high tides and low tides according to their relative height. Thus, we have the the low low, the low high, the high low and the high high tides from midnight until midnight.
So far, we have taken for granted this concept of a tide height. But what is the height of a tide and how do we figure it out? The tide height is computed from a standard tide height called the zero tide height. The zero tide height is the average height of all the low tides or lowest tides in a given period of time for a given location. Without going into all the details, suffice it to say that there is a zero tide height from which all the other tide heights can be gauged.
In the figure above, the zero tide height is indicated by a white line. Tides above this line have positive values for their heights and tides below this line have negative vales for their height. For example, what is the height of the low low tide for this day? Because the dark blue graph touches the zero line at the low low tide, the height of this tide is zero feet. Using the same process, calculate the height of the lo hi, hi lo and hi hi tides. You should be able to see that they are slightly less than 4 feet, a little more than 2.5 feet and a little more than 5 feet, respectively.
Note also that the times of the 4 tides are given at the top of the graph. For example, what is the time of the high low tide? It is 1:12 PM.
Another useful way to determine the times of tides is a tide table. The time and height of the tides are listed in columns for each date. What is the time of the hi hi tide on March 11? What is the height of the lo hi tide? What is the time and height of the lo lo tide? When is the hi lo tide?
Did you note that the times given are in GMT? What are the local times for the tides you just identified?
Make sure you can identify these correctly. Use the links to tide tables and make sure you can figure out the different tides, their heights and their times. Many people think this is easy but when I give exam questions on it, too many people miss the questions.
Note that the low low tide on March 16 (at 22:36) is a minus tide, i.e. -0.7 feet. That means that sea level at the low low tide is 0.7 feet below the zero tide height.
Calculate the tidal range for this day. Recall that the tidal range is the difference between the highest and lowest tide of the day. We've already said that the low low tide is -0.7 feet. Looking at the tide table, we can see that the high high tide (at 16:01) is 5.8 feet. Setting up the calculation:
Height (hi hi) - Height (lo lo) = Tidal Range, or
5.8 feet - (-0.7) feet = ????
If you did not get 6.5 feet, then check again. Recall that when you subtract a negative, you add. Thus,
5.8 + 0.7 = 6.5 feet.
Here's an easy way to think about it. Imagine you are standing on the beach. Towards the land, another person is standing 5.8 feet away. Below you, towards the ocean, another person is standing 0.7 feet. What is the distance between the two people? Take yourself away and measure and you find that 6.5 feet is the distance between them. Try it with pencils and different positive and negative values.
Knowing the time and height of tides can make your beach-going experience much more pleasurable. If you want to see the many fascinating organisms living on rocky shores, you will want to visit during the lowest tides. If you are hiking along the coasts (see the article in Backpacker this month), you will need to calculate the times and heights of the tides to gain access around the headlands. If you own a boat, you will want to know the time and height of the tides to make sure you stay afloat in whatever location you are boating. If you are a surfer, the time and height of the tides are vitally important for the type of waves you will surf at any given location.
There are many practical reasons for knowing something about the tides. With a little practice, it can be quite fun reading a tide calendar.