Introduction to Tide

Tide, the rhythmic rising and falling of the surface of the oceans, seas, and other bodies of water. (Similar rhythmic movements, also called tides, occur in the earth's crust and atmosphere, but these movements can be detected only with sensitive scientific instruments.)

When the water flows in toward the land, it is at flood tide; the highest level it reaches is high tide. The water recedes during ebb tide; the lowest level it reaches is called low tide. The part of the shore between the high-tide line and the low-tide line is the intertidal zone. Here live plants and animals adapted to survive both on land and underwater. Barnacles, kelp, and some forms of crabs and clams live in the intertidal zone.

In most areas, there are two high tides and two low tides in approximately a day. Tidal movement occurs over the entire area of any large body of water. It is noticeable, however, only where the water's rise and fall can be measured against the land.

Tides have been carefully observed because of their importance in navigation. Most of the world's harbors are affected by noticeable tides. The depth of submerged hazards, the amount of water in channels, and the direction of the current, which all depend at least in part on the tide, affect the safety of ships passing into and out of the harbors.

Tides vary greatly from place to place around the world. Tide predictions for a specific area must therefore be based on an extensive series of observations and measurements in that area. Even so, tide predictions are not completely accurate because variable factors, such as weather conditions, have a measurable effect on the tides. Tidal information is published in the newspapers of coastal cities, in almanacs, and in tide tables used aboard ships.

The rising and falling water of the tides possesses great kinetic energy (the energy of motion). Part of this energy is expended against the shores. The erosion of shorelines is due in part to the tides. In a few countries, including France and Canada, the tides have been used to generate electricity. .)

In some rivers the tide produces a tidal bore. Tidal waves are not in any way related to tides. These destructive waves are caused by undersea earthquakes and volcanic eruptions or by storms at sea.

The Causes of Tides

Three principal forces are involved in the production of tides: (1) gravitational attraction between the moon and the earth; (2) gravitational attraction between the sun and the earth; and (3) the force of the earth's gravity, which pulls every particle of the earth toward the earth's center. The moon is mainly responsible for the tides (its effect is about 2.2 times as great as the sun's).

It is useful to consider the tides as they would be if the earth were a smooth ball uniformly covered by water, ignoring temporarily the influence of the sun and such modifying factors as water depth, size of the bodies of water, and the shape of shorelines. On such a ball two tidal bulges would form as a result of the moon's gravitational attraction—one directly under the moon and one on the opposite side of the earth.

The strength of the gravitational attraction between any two bodies is described by Newton's Law of Gravitation. Every particle of the earth is attracted by (and attracts) the moon. The force of this attraction is proportional to the product of the particle's mass and the mass of the moon and inversely proportional to the square of its distance from the center of the moon. (The average of the distances of all the moon's particles from any point on or within the earth is approximately equal to the distance of the moon's center from that point. Therefore, the moon's entire mass can be thought of as concentrated at its center.)

The difference between the attraction of the moon for a particle at any given point on earth and its average attraction, per particle, for all the particles of the earth is called a differential force. (The average attraction per particle is equal to the moon's attraction for a particle at the center of the earth.) The differential forces vary in direction and strength over the earth.

The key to the production of two tidal bulges is in the direction in which the differential forces act. On the side of the earth toward the moon, where the attraction on each particle is greater than average, the differential forces are directed toward the moon. On the side of the earth away from the moon, where the attraction on each particle is less than average, the differential forces are directed away from the moon.

The differential forces directed toward the moon pile up the water into a bulge under the moon, while those directed away from the moon pile up the water into a bulge on the opposite side of the earth. The loss of water to the two bulges creates the low tides elsewhere; the bulges themselves are the high tides.

Contrary to popular belief, the tidal bulges are not the result of a lifting of the water. The moon, for example, does not pull up the water directly under it. The earth's gravity is much stronger than the moon's attraction. However, over most of the earth, the differential forces are directed at an angle to the surface. An angular force can be divided into a horizontal component and a vertical component. The vertical component of the force is ineffective because of the much stronger opposing pull of gravity. However, the horizontal component, which is unopposed, is effective in moving the water. The water slides over the surface of the earth and piles up into the two tidal bulges.

The bulges hold their positions with respect to the moon as the moon revolves around the earth and the earth rotates on its axis. The basic lunar cycle at any point on earth therefore consists of two high tides and two low tides during every 24 hours and 50 minutes.

The strength of a differential force at any given point on earth is proportional to the mass of the celestial body exerting the gravitational attraction and is inversely proportional—approximately—to the cube of the distance between the celestial body's center and that given point. (This inversecube relationship between distance and differential force explains the sun's relatively small effect on the tides despite its great mass.)

The sun produces a regular modification of the lunar tides. The action of the sun on the earth's waters is similar to that of the moon except that the basic cycle of solar tides is 24 hours.

The relative positions of the sun, the moon, and the earth vary throughout a lunar month. When the three are in a nearly straight line, the sun's effect is added to the moon's, producing higher high tides and lower low tides than usual. These tides, called spring tides, occur at the times of the new moon and full moon. When the moon is in first or third quarter, the sun and the moon are 90° apart in the sky. The attraction of the sun then works against that of the moon, and the range between high and low tides is less than usual. These reduced tides are called neap tides. Two spring tides and two neap tides occur in each lunar month.

Variations In the Tides

The basic lunar tides with spring and neap tide modification are subject to further, more complex variations. Some of these variations are based on long-term cycles in the relative positions of the sun and the moon. One such cycle is about 18 years long and produces greater than usual spring tides. Other variations in the tides cause different tidal patterns to exist in different places on the earth at one time. In the Mediterranean Sea the range from high tide to low tide is usually less than three feet (0.9 m). The highest tides in the world occur in part of the Bay of Fundy, between New Brunswick and Nova Scotia, Canada. There, during spring tides, the water rises more than 50 feet (15 m) from low tide to high tide.

The earth's orbit around the sun is elliptical, as is the orbit of the moon around the earth. There are regular changes in the relative abilities of the sun and the moon to raise tides. The position of the moon above and below the earth's equator varies during the month, and the sun's position shows similar variation during the year. Because of these variations in position, the tidal bulges occur at different locations at different times.

On earth, factors such as the depth and breadth of the bodies in which tides occur and the configuration of shorelines affect the tides. Tides are also modified by the friction of the water against sea bottoms. The various combinations of all the factors affecting tides produce three distinct tidal patterns—semi-diurnal, diurnal, and mixed.

Semi-diurnal (semi-daily) Tides

have two nearly equal high tides and two nearly equal low tides during a period of 24 hours and 50 minutes. High tides tend to follow one another at intervals of 12 hours and 25 minutes. The Atlantic Ocean has primarily semidiurnal tides.

Diurnal (daily) Tides

occur where there is only one high tide and one low tide a day, with high tide occurring 24 hours and 50 minutes after the preceding high tide. The Gulf of Mexico has primarily diurnal tides.

Mixed Tides

occur where there are two high tides and two low tides of very unequal size in each period of 24 hours and 50 minutes. San Francisco Bay, for example, has a mixed tide pattern of very high tide, very low tide, moderately high tide, moderately low tide. The Pacific Ocean has primarily mixed tides.