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 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
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
Moon
Gravitational
force
– 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
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 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>.