Tides November 18-19, 2009

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

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 CanadaBay 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 –AnimationAnimation –Blue planet video segment

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

Gravity, Inertia, and the Two Bulges 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 Earth Moon Gravitational force Inertia

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

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)

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

Tidal Variation due to Variation in Moon and Earth Orbits.

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

Tidal Cycles of the World

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

Tide Resources McNish, Larry. "RASC Calgary Centre - A Complete Guide to." 17 Dec Oct "Tides and Water Levels." NOAA Ocean Service Education. 25 Mar National Oceanic and Atmospheric Association, U.S. Department of Commerce. 29 Oct 2008.