TIDAL POWER!

Transcription

1 Tide Notes terms: spring tide, neap tide, flood tide,ebb tide, slack tide, mean low water, mean high water, sea level, diurnal tides, semidiurnal tides TIDAL POWER! Where are the highest tides in the world? The Guinness Book of World Records states the world's highest tides to be in the Minas Basin, NS, with the maximum tidal range recorded at 16.8 meters (54.6 ft). Parrsboro, the largest Seaport on the Minas Basin, affords the best view of this tidal phenomenon. At this point the tide floods and ebbs over 3.2 kilometers (2 mi.) of tidal flat from the low watery mark to the head of the harbor. Each phase of the cycle takes approximately 5hrs\40 min. which results in each succeeding high or low water mark range an average 14m. (45.5 ft.) while the harbor heights are about 7.5 m (24 ft.). For more details see the websites and Tide Amplitudes Around the World

2 Place Average Tidal Range in Meters Portland ME 2.7 Boston MA 2.9 NYC 1.4 Savannah GA 2.2 Key West FL.4 Galveston TX.4 San Diego CA 1.2 San Francisco CA 1.2 Seattle WA 2.3 Cordova AK 3.0 Honolulu HI.4

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4 TIME LAPSE VIDEO BAY OF FUNDY

5 The Canadian Press

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7 The tide generating forces act on every particle of the Earth but only the water is free to move. Thus on the side of the Earth nearer the Sun or Moon, gravitational attraction of the Sun/Moon combines with the centrifugal force of the Earth and the water tends to pile up and cause a high water at position "a", as shown in Fig. A. On the opposite side of the Earth, centrifugal force is greater than the gravitational attraction and tends to cause another high at "b". At the meridian pp' the tendency is for low water to be caused. Figure A

8 The Earth makes one complete revolution about its axis in an average of 24 hours, so that any position on the Earth is in direct line with the Sun every 24 hours. The interval between successive high waters caused by the solar tidegenerating effect is thus on an average 12 hours. Because of the Moon's movement in space any position on the Earth is in direct line with the Moon once every lunar day, which is on average about 24 hours 50 minutes. The interval between successive high waters caused by the lunar tide-generating force is thus on average 12 hours 25 minutes. SPRING TIDE Approximately 15 days later, at the full Moon, the Earth is between the Sun and Moon as shown in Fig. D and the Moon and Sun are acting in opposition.

9 SPRING TIDE NEAP TIDE

10 Every place on Earth passes through these points, called high tides, twice per day as the Earth rotates. High tides occur every 12 hours 25minutes remember, the Moon moves! The Sun s tidal effect on Earth is not as strong. the ratio Earth s diameter : distance to Sun is much less than ratio Earth s diameter : distance to Moon What are the different types of Tides When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season. When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides. Spring Tides When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides (they do not have anything to do with the season Spring). They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon

11 Neap Tides During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons. This fight between Moon s pull & Earth s rotation causes friction. Earth s rotation slows down (1 sec every 50,000 yrs.) Synchronous Rotation is when the rotation period of a moon, planet, or star equals its orbital period about another object.

12 Tidal friction on the Moon (caused by Earth) has slowed its rotation down to a period of one month. The Moon now rotates synchronously. We always see the same side of the Moon. Tidal friction on the Moon has ceased since its tidal bulges are always aligned with Earth. A FEW FACTS ABOUT LUNAR TIDES The gravitational force of the moon is one ten-millionth that of earth, but

13 when you combine other forces such as the earth's centrifugal force created by its spin, you get tides. The sun's gravitational force on the earth is only 46 percent that of the moon. Making the moon the single most important factor for the creation of tides. The sun's gravity also produces tides. But since the forces are smaller, as compared to the moon, the effects are greatly decreased. Tides are not caused by the direct pull of the moon's gravity. The moon is pulling upwards on the water while the earth is pulling downward. Slight advantage to the moon and thus we have tides. Whenever the Moon, Earth and Sun are aligned, the gravitational pull of the sun adds to that of the moon causing maximum tides. Spring tides happen when the sun and moon are on the same side of the earth (New Moon) or when the sun and moon are on opposite sides of the earth (Full Moon). When the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker; these are called neap tides. Spring tides and neap tide levels are about 20% higher or lower than average. Offshore, in the deep ocean, the difference in tides is usually less than 1.6 feet The surf grows when it approaches a beach, and the tide increases. In bays and estuaries, this effect is amplified. (In the Bay of Fundy, tides have a range of 44.6 ft.) The highest tides in the world are at the Bay of Fundy in Nova Scotia, Canada. Because the earth rotates on its axis the moon completes one orbit in our sky every 25 hours (Not to be confused with moon's 27 day orbit around the earth), we get two tidal peaks as well as two tidal troughs. These events are separated by about 12 hours. Since the moon moves around the Earth, it is not always in the same place at the same time each day. So, each day, the times for high and low tides change by 50 minutes. The type of gravitational force that causes tides is know as "Tractive" force.

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15 Tidal Differences Table Name: Class: Procedure TIDAL DIFFERENCES PLACE Time (hrs.:mins.) Height (ft.) HW LW HW LW York River Tues Marshes Light 0:09 0: Gloucester Point +0:16 +0: Yorktown +0:07 +0: West Point +2:03 +2: James River Mulberry Point +1:56 +2: Hog Point +2:11 +2: Jamestown Island +2:54 +3: Outer Coast of Virginia Virginia Beach 1:30 1: Chesapeake Bay Eastern Shore Onancock +2:52 +3: Fisherman s Island 0:47 1: Enter the date and location. Telling Tides Worksheet Date: 2. In the Hampton Roads column, enter the times and heights of the tides for the selected date, as found on the Tide Calendar. 3. In the Tidal Differences column, enter the corresponding figures from the Tidal Differences Table. Be sure to indicate + or. 4. Sum the figures (either add #1 and #2, or subtract #2 from #1. Watch the + and signs!) in the Hampton Roads and Tidal Difference columns to complete the Corrected Tides column.

16 Name: Time of first high tide Height (ft.) of first high tide above mean low water Time of first low tide Height (ft.) of first low tide below mean low water Time of second high tide Height (ft.) of second high tide above mean low water Time of second low tide Height (ft.) of second low tide below mean low water #1 Hampton Roads Date: Location: #2 Tidal Difference #3 Corrected Tides Our Famous Tides We have included 3 examples of high/low tide images taken at the exact same location in order to present some perspective for people who have never experienced the tides of the Bay of Fundy first hand. Example 1

17 High Tide Low Tide Example 2

18 High Tide Low Tide

19 Example 3 High Tide Low Tide

20 Anchorage Alaska boasts the world's second highest tides: varying over 40 feet, low to high tide! Bore tide (one of the three highest in the world, and a weird phenomenon: capillary action on a gigantic scale!) occurs 2 hours 15 minutes after low tide; best viewed between Mileposts 101 and 90 Seward Highway (26 to 37 miles from Anchorage). What is a tidal bore? A tidal bore (bore = crest or wave) is a natural phenomenon which is seen in a very few parts of the world. The Bay of Fundy is particularly noted for its extremely high tides, the highest in the world, and for its tidal bores. A tidal bore near Chignecto Bay, New Brunswick

21 A tidal bore is a wall of water that moves up certain low-lying rivers due to an incoming tide. Tidal bores form when an incoming tide rushes up a river, developing a steep forward slope due to resistance to the tide's advance by the river, which is flowing in the opposite direction. Thus we have the phenomenon of the river changing its flow before your very eyes, flowing in OVER the outgoing river water. The height of the tidal bore increases with the range of the tide and may very in height from just a ripple to several feet. Some links for more info... Tidal bore rafting.