tide

The rhythmic rise and fall of the oceans water at a fixed location is known as the tide. Changing water levels create hardships for coastal organisms by exposing them. Tides are long waves moving through the ocean. When the crest of the moving tide reaches a location, high tide occurs. Low tide is when the trough reaches the location.

Best known as the rise and fall of these around the edge of land, the tides are caused by the gravitational attraction between the earth and the sun and between the earth and the moon. While tides go unnoticed far out at sea, they are easily observed along the shoreline.

Tides behave differently in different places. In some coastal areas there is a regular pattern of one high and one low tide each day known as a diurnal tide. In other areas there is a high-low water sequence repeated twice a day...semidiurnal tide and these tides usually reach about the same level at high and low tides each day. The third type of tide has two high and low tides a day but the tides reach different high and low levels during a daily rhythm. This is called a semidiurnal mixed tide. It is caused by a diurnal (daily) inequality by combining a semidiurnal and diurnal tide. Tides are charted on a marjoram which shows the height of the tide above mean low water (sea level) and the time of the peaks of the high/low tides.

In the uniform tidal system (semi and diurnal) the greatest height to which the tide rises on any day is known as high water and the lowest point is low water. In a mixed system it refers to higher high and lower high water and higher low and lower low waters.

Tidal observations made over a period of time are used to calculate the average or mean tide levels.

Since the depth of coastal waters is important for navigation, an average low-water reference is established: depths are measured from this level and recorded on navigational charts. This area is usually established at mean low water and the zero reference or tidal datum, is established at this point. In mixed tidal areas, mean lower low water is used as the tidal datum. Sometimes the low tide level may fall below the mean value used as the tidal datum producing a minus tide.

When the tide is rising, it is called a flooding or flood tide and when its falling it is an ebbing tide.

Currents associated with the rising and falling of the tide in coastal waters is a tidal current. They can be very swift and dangerous. When the tide turns, reverse direction, there is a period of slack water during which the tidal currents slow and then reverse.

0ceanographers analyze tides in two ways. One is using universal laws of physics called equilibrium tidal theory and the other is studying them as they occur in nature called dynamic tidal analysis which study the oceans tides as they occur modified by the land masses, geometry of the ocean basins and earth's rotation.

The effect of the sun and moons gravity on tides is easily explained using the equilibrium theory. 9.3 A water covered planet with its satellite moon orbits the sun. The moon is held in orbit about the earth by the earth's gravitational force (B). There is also a force pulling the moon away from the earth and send it spinning into space (B') and is considered centrifugal force in this discussion. The earth and moon are held in orbit by the gravitational attraction between the center of mass between the moon and earth system and the sun (A). The centrifugal force pulls the center of the mass away from the sun (A'). To stay in orbit A must equal A'. (like twirling a string with a weight on it around your head and the string is gravitational attraction.

In the earth moon sun system, the mass of the sun is great but its very far away and while the mass of the moon is small, its closer and therefore has a greater attractive effect on water particles than does the sun, so the moons effect will be considered first because its greater.

Water particles on the side of the earth facing the moon are closest to the moon are acted on by the larger moon gravitational force and because water is liquid and deformable, the force is applied to water particles toward a point directly under the moon. It produces a bulge in the water covering. At the same time, the centrifugal force of the earth moon system acting on the water particles at the earths surface opposite the moon creates a bulge too. Now a model would show 2 bulges on opposite sides of the earth but that is on a uniformly deep water covered planet.. The area between bulges (opposite each other) would be low water areas or depressions (or 2 crests and two troughs--high tide and low tide).

The moon and earth are moving in the same direction along its orbit during a 24 hr period but the earth must turn an extra 12' or 50 minutes for the moon to be directly over the same place. Therefore the tidal day is not 24 hours long but 24 hr and 50 minutes explaining why tides arrive at a location about an hour later each day.

The equilibrium tides produced are semidiurnal or two high and two lows a day. The tidal distortion of a models water covering produce a wave form known as the tide wave. The crest of the wave is high water ( tide) and the trough is low water (tide). The wavelength of this wave is 1/2 the circumference of the earth and period 12 hr and 25 min.

While the moon plays a greater role in tide producing, the sun also produces its own tide wave. Though it is large, the long distance away means its tide raising force is only 46% that of the moon and the average time is 24 hours not 24h50m. The tide wave produced by the moon is greater magnitude and continually moves eastward relative to tide wave produced by the sun and therefore the tide forces produced by the moon are greater and more important than that of the sun (therefore tidal day 24h50m).

During the 29 1/2 days it takes the moon to orbit the earth, the sun, the earth and the moon move in and out of phase with each other. During the period of the new moon, the moon and sun are lined up on the same side of the earth so that the high tides, or bulges, produced independently of each, coincide. Since the water level is the result of adding the two wave forms together, tides of maximum height and greatest depression, or tides with the greatest range between high and low water are produced. These are spring tides. The vertical displacement of the tide may also be described as amplitude or 1/2 the range (distance above or below sea level.)

In a weeks time, the moon is in its 1st quarter and moved about 12' per day, until its at a right angle to the sun wave. Now the crests of the moon tide will coincide with the troughs produced by the sun and the same is true of the sun's crests and moons troughs. They tend to cancel each other out and the range between high and low water is small. These are low-amplititude neap tides.

At the end of another week, the moon is full and the sun, moon and earth are again lined up, producing crests that coincide and tides with the greatest range between high and low waters, or spring tides. These are followed by neap tides seven days ;later and the cycle, 4 weeks, continues with a spring tide every two weeks...etc. This 4 week progression creates a tidal cycle of changing tidal amplitude.

If the earth and moon are aligned so that the moon stands north or south of the earth's equator, one bulge is in the Northern Hemisphere and one in the Southern Hemisphere. A point in the middle latitudes passes through only 1 crest and one trough during the tidal day. This type of diurnal tide is called a declination tide, because the moon is said to have declination when it stands above or below the equator.

The sun also influences these tides as it sits over 23.5N/S at summer and winter solstice and the variation causes the bulge created by the sun to oscillate north/south (making a more diurnal sun tide during winter and summer. The moons declination is at 28.5N/S and because the orbit is inclined 5' to the earth sun orbit, it takes 18.6 yrs for the moon to complete its cycle of max. declination. When the sun and moon coincide, both tide waves become more diurnal. Also the moon doesn't move around the Earth in a perfectly circular orbit or even does the earth circle the sun at a constant distance. In the Northern Hemisphere, the earth is closer to the sun so the solar tides play a greater role as a tide producer in the winter than summer. see p254 tide charts

Because of all the combinations, its not possible to predict the earths tides from our knowledge of tide raising bodies alone. But with a combination of actual local measurements with known astronomical data, tide predictions are quite accurate.

Water level recorders are installed at coastal sites and the rise and fall are measured over a period of years...at least 19 years are needed to allow for the long 18.6 year period of declination of the moon. Tide Tables are published annually by NONA. The tables give the dates, times, and water levels for high and low water at a primary tide station. There are only 196 of these but consulting a list for other 6000 auxiliary stations applying corrections to the times and heights of the primary stations helps more localities get accurate tide predictions.

Tidal predictions are based on recorded high measurement from past records which are used in the future.

Like tides, these currents are measured at primary locations and data is published similar to tide tables. Useful for moving in and out of estuaries.

Summary
Without land, 2 bulges , 1 on side of the earth closest to the moon, 1 on the other side would appear. Bulge is high tide from mutual attraction between earth and moon. Moon gravitational attraction pulls and moves the water. The opposite bulge, centrifugal (I know, there is no such thing, ) force created as the earth and moon revolve around a common point...barycenter-4670 km (2900miles) from earths center.

Tides occur at different times because the moon goes around the earth in 24h50m or 1 lunar day. (freq 12h25m)

Due to friction between earth and tidal bulges, High tide is given about 50 minutes after the moon passes over that point. known as a lunar semidiurnal tide.

Vertical height ...high to low water is the tidal range and varies day to day because of the sun and the moon. Solar tides are 1/2 the size of lunar tides. When the sun and moon line up (2x per month) tides are higher and lower than usual...spring tide. When the moon is at right angles to the sun, there is less gravitational effect that lessens the tidal range (neap tides) Between these tides the height varies throughout the month.

Differences in tidal frequencies and range occur because of the shape of the ocean basins and coastline (Hawaii is a few cm while the Bay of Fundy is 20m.

11. Tide readings

1. What is a tide?
2.. How much stronger is the gravitational force of the moon on the earth than the sun on the earth?
3. What is the average interval between tides?
4 What is a amphidromic point?
5 How does shallow water effect tides?
6. Where do diurnal tides occur?
7. What is a cause that effects the range of tides?