1. Tides November 18-19, 2009

2. What are Tides? • A tide is defined as a periodic rise and fall of the sea surface • very-long period waves noticeable only at the shoreline • originate in the open ocean • High tide – wave crest reaches the shoreline • Low tide – wave trough reaches the shoreline

3. Tidal Range & Tidal Currents • Tidal range - the difference in water height between high and low tides • varies from a few cm to up to 48 feet (14 m) at the Bay of Fundy in Canada • Tidal current – horizontal movement of water that accompanies the rising and falling tide • incoming current is a flood current • outgoing is an ebb current • strongest during high or low tides, weakest in between the two • Animation • Blue planet video segment

4. What Causes Tides? • Result from the gravitational pull of the moon and sun on the Earth. • Newton’s law of universal gravitation • More massive an object, greater its gravitational pull • Varies inversely as the square of the distance between objects • Tide generating force • vary inversely as the cube of the distance from the tide generating object. • Therefore the moon has a greater effect on the tides due to its proximity

5. Gravity, Inertia, and the Two Bulges Moon • Two tidal bulges (high tides) form • Bulge on the moon side • caused by gravitational attraction of moon and Earth exerted on the fluid (oceans) • largest bulge • overcomes inertia of water • Bulge on the “far side” of the Earth opposite the moon • caused by inertia – the tendency of moving objects to continue moving in a straight line • inertia exceeds gravitational force here Gravitational force Earth Inertia

6. Changing Angles and Changing Tides • Moon revolves around the Earth • Its declination - angle relative to the equator - increases and decreases • Varies the height and intensity of tides • Monthly variation • Animation

7. Changing Angles and Changing Tides • Because of Earth’s tilted axis, the sun’s relative position (declination) to the equator changes throughout the year • Minimum (spring/fall equinoxes) • Maximum (summer/winter solstices)

8. Frequency of Tides – The Lunar Day • Lunar orbit: 29.5 days • Lunar tide • Solar tide (1/2 as large as lunar one) • When both in alignment spring tides occur. When moon not in alignment, neap tides. • Animation

9. Tidal Variation due to Variation in Moon and Earth Orbits.

10. Continents in the Way .. Again Land masses can create three different tide patterns • Diurnal • Semidiurnal • Mixed semidiurnal

11. Tidal Cycles of the World

12. Other factors affecting tides… • Shoreline and coastline topography • Shape of bays and estuaries • Local wind and weather patterns

13. Tide Resources McNish, Larry. "RASC Calgary Centre - A Complete Guide to." 17 Dec 2007. 29 Oct 2008 <http://calgary.rasc.ca/radecl.htm>. "Tides and Water Levels." NOAA Ocean Service Education. 25 Mar 2008. National Oceanic and Atmospheric Association, U.S. Department of Commerce. 29 Oct 2008 <http://www.oceanservice.noaa.gov/education/kits/tides/welcome.html>.