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OC103 Lesson 20 Tides What are Tides A tide is a broad single wave that stretches across large areas of an ocean basin Tides occur due to the gravitational pull of the Moon and Sun on the ocean water In terms of the characteristics that we used to describe other kinds of waves the key features of tides are High tide Wave crest Low tide Wave trough Tidal range Wave height which for tides can be 1 m to 15 m Tidal period Wave period which depending on the location is either 12 hours and 25 minutes or 24 hours and 50 minutes For example the plot below shows the predicted tides for Newport Oregon for 7 days around a Memorial Day weekend The tides are essentially very broad waves with crests and troughs that pass the Oregon Coast about twice per day Notice from the plot that the tidal period at Newport is 12 hours and 25 minutes that is it is 12 hrs and 25 min between 2 high tides or between 2 low tides so it takes slightly longer than 1 day to go through 2 full tidal cycles in this area see how the first low tide of each day occurred at midnight on the 1st day on this plot but gets later and later each night 50 minutes later to be exact until it occurs at about 5AM on the last day on this plot Notice also that the tidal range the height difference between high and low tides varies from just over 2 feet early on May 27th between 4 and 6 feet above Mean Lower Low Water to almost 12 feet early on June 2nd between 9 feet above and 2 feet below MLLW Tides are caused by a combination of factors but they are highly predictable once we have about a year of observations in a location In explaining what causes tides we will also explain why tidal ranges and periods vary with location i e why some locations have large ranges and some small and some locations have only 1 tidal cycle per day while most have 2 and also why the tidal range at any single location varies predictably from day to day Origin of the Tides Unlike wind driven surface waves tides are caused by two principal