• 1) Sea level is not constant but varies over time (usually 1 - 5 m/day):

  • regular cycles are observed daily (12hr 25min and 24hr 50min), bi-weekly (14 d) and half-yearly (6 mo)
  • three common patterns are:

    • diurnal tides (1 high and 1 low per day),
    • semi-diurnal tides (2 similar highs and 2 similar lows per day),
    • mixed, semi-diurnal tides (2 dissimilar highs and 2 dissimilar lows per day)

  • tide tables existed by 1213 AD, but mechanism not clear until Newton's Principia Mathematica in 1687, which outlined the roles of gravitational and inertial forces

• 2) 12 hr 25 min cycle (tidal bulge) due to: a) gravitational attraction of moon, b) inertial ("centrifugal") force of earth-moon system, and c) motion of the moon in the same direction as earth's rotation

• 3) 24 hr 50 min cycle is most pronounced at mid latitudes and is due to the inclination of earth's axis (23.5 degrees)

• 4) 14 day cycle due to joint effects of earth, moon and sun; yields 'spring' (max. amplitude) and 'neap' tides (min. amplitude)

• 5) 6 month cycle (maximum amplitude spring tides per month) also most pronounced at mid latitudes; due to joint effects of earth, moon, sun and inclination of earth's axis; greatest at solstices

• 6) Predicted tidal patterns differ on different coasts:

  • ocean depth: 'tidal bulges' are really giant waves (friction with bottom has slowed earth's rotation approx. 0.5 hr/100MY)
  • ocean size: larger basins have more water to move
  • coast configuration: 'seiching' in a basin can amplify or damp tides (e.g., Bay of Fundy has 15m! tides and tidal bores)

• 7) Local predicted tidal patterns may also be disrupted by:

  • atmospheric pressure: local low pressure can cause 'storm tides'
  • winds: can pile up water along, or pull it away from, coasts

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