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THE INSIDE STORY: TIDES Robert Perry Malibu High School What are TIDES ? Tides are the regular and predictable periodic variations in sea level on the Earth that correspond to changes in the relative positions of the Moon and the Sun. High tide Low tide High tide Low tide Interval between successive high tides = 12 hours, 25 minutes Q. How long between high tide and low tide ? What is the #1 cause of ocean tides? (hint) Spring tides gravity gravity Neap tides gravity gravity Isaac Newton (1642 -1727) was the first person to explain tides scientifically. http://astrosun.tn.cornell.edu/courses/astro201/moon.htm Water tends to accumulate on the parts of the Earth's surface directed toward and directly opposite the Moon. Neap Tide High tides and low tides are not extreme. NEAP TIDES. High tides and low tides are of minimal range. SPRING TIDES. High tides and low tides are extreme. (Calendar courtesy of Tidelines, Inc) Spring Tide High tides and low tides are extreme. Moon + tide graphics courtesy of US Navy http://pao.cnmoc.navy.mil/educate/neptune/trivia/physical.htm Spring Tide High tides and low tides are extreme. Perigean tides A perigean tide refers to a tide that occurs when the moon is closest to the earth. The moon's orbit around the earth is elliptical rather than circular, which means that the distance between earth and moon is always changing. Perigee refers to the time when the moon and the earth are closest to one another. At perigee, the moon is about 30,000 miles closer to earth than at apogee, when the moon is farthest from earth. Perigee is reached about once a month, roughly the time it takes for the moon to revolve around the earth. When the moon is closest to earth, its effect on tides is greatest. Woods Hole Sea Grant Source: builtstronger.com How might tides combine with storm surf to cause property damage ? tide Adapted from Woods Hole Sea Grant Types of tides SEMIDIURNAL MIXED DIURNAL Semi-diurnal tides have two high and two low waters per tidal day. They are common on the Atlantic coasts of the United States and Europe. Mixed tides have a higher high water and lower high water as well as higher low water and lower low water. The tides around west coast of Canada and the United Sates are of this type. In parts of the northern Gulf of Mexico and Southeast Asia, tides have one high and one low water per tidal period. (Calendar courtesy of Tidelines, Inc) Q. What type of tide do we have here in southern California ? (Calendar courtesy of Tidelines, Inc) Q. What type of tide do we have here in southern California ? A. mixed tide. How do tides affect marine life at the beach ? High tide Organisms covered by water. Low tide Organisms exposed to air. Exposure to air causes bands or zones of life along the beach. Intertidal Zone Total Exposure to Air Hours per Year Zone 1 High Intertidal 7,200 - 8,760 Zone 2 Upper-Middle 3,200 - 7,200 Zone 3 Lower-Middle 400 - 3,200 Zone 4 Low Intertidal 0 - 400 Data modified from Hedgepeth, Ricketts and Calvin (1968). Intertidal Zonation The clusters or bands formed by communities of organisms living within certain tide ranges along the shore. Southern California rocky shore dominant species: Number: 1 Name: Spash Species: snails 2 Upper-mid barnacles 3 Low-middle mussels 4 Low algae snails barnacles mussels algae Intertidal zones at La Jolla, California Stephenson & Stephenson (1972) Intertidal zonation patterns on a rock breakwall in North Carolina. Stephenson & Stephenson (1972) Zonation on the west coast of South Africa Stephenson & Stephenson (1972) Intertidal zonation patterns along the Florida Keys. Stephenson & Stephenson (1972) CONCLUSION: Alternate exposure to air and water caused by the tides results in intertidal organisms occupying specific zones according to their range of tolerance to dryness, temperature, predation, feeding, respiration and reproduction. This pattern can be observed world wide. THE END This slide presentation was prepared for UCLA OceanGLOBE participants. Images and materials may be used outside of the UCLA OceanGLOBE program for non-profit, educational purposes only. Send comments to: [email protected]