Understanding Tides: Forces, Patterns, and Phenomena

1. Tides Fig. 10-6 Fig. 10-7

2. Tide-producing forces • Gravity and motions among Earth, Moon, and Sun Fig. 10-2

3. Centripetal force “tethers” Moon to Earth • Center-seeking force • Directed away from barycenter

4. Resultant tidal forces • Gravitational force and centripetal force • Resultant force moves ocean water horizontally Fig. 10-6 Fig. 10-7

5. Tidal bulges • Two equal and opposite tidal bulges • Earth rotates beneath tidal bulges • Two high tides and two low tides per day Fig. 10-8

6. Complications to simplest equilibrium theory • Oceans do not cover entire Earth • Oceans do not have uniform depth • Friction between ocean and seafloor • Continents • Moon not always in same place with respect to Earth • Lunar day longer than solar day

7. Lunar day • 24 hours and 50 minutes • Moon revolves around Earth • Earth has to “catch up” with Moon to reach same position Fig. 10-9

8. Time between successive high tides shifts day after day • Moon rises later each successive night

9. Solar tidal bulges • Tide-producing force of Sun is smaller • Sun much farther away

10. Month tidal cycle • Spring tides • New Moon, Full Moon • Higher than usual high tides Fig. 10-12

11. Neap tide • First Quarter, Last Quarter • Lower than usual high tide Fig. 10-12

12. Declination of Sun and Moon • Orientation of Sun and Moon is never directly over the equator • Sun 23.5o N and S, yearly cycle • Moon 28.5o N and S, monthly cycle • Unequal tides • Successive tides different tidal range

13. Unequal tidal range Fig. 10-15

14. Elliptical orbits • Perigee • Lunar tidal force greater • Higher high tides • Apogee • Lunar tidal force lesser • Lower high tides Fig. 10-16

15. Dynamic theory of tides • Tide shallow-water wave • Speed varies with depth • Lags behind Earth’s rotation • Rotary flow in open ocean basins • Amphidromic point – ocean tides form cells • Cotidal lines – indicate time of high tide

16. Rotary flow • Crest (high tide) rotates • Counterclockwise in Northern Hemisphere • Clockwise in Southern Hemisphere

17. Tidal patterns • Diurnal • One high, one low tide per lunar day • Period of tidal cycle 24 hours 50 minutes • Semidiurnal • Two high, two low tides per lunar day • Period 12 hours 25 minutes • Equal range

18. Mixed • Two high, two low tides per lunar day • Unequal range • Most tides are mixed

19. Standing waves • Forced standing wave caused by tides • Free-standing waves caused by strong winds or seismic disturbances Fig. 10-22

20. Bay of Fundy • Largest tidal range (spring tide max 17 m) • Shape of basin • Oscillation period close to tidal period • Shoals and narrows to north • Basin oriented toward right (Coriolis moves water toward right) Fig. 10-24

21. Tidal bores • Wave created by tide rushes upstream • Large tidal range • Low-lying coastal river • Max 8 m high Fig. 10A

22. The video clip shows tides in the Cape Cod Estuary, Massachusetts

23. The video clip shows tides on Cape Cod Beach, Massachusetts.

24. In thevideo clip ; Dr. Hoffman, from Old Dominion University, explains the tides.

25. Tidal Calendars • http://www.tidelinesonline.com/