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What Are Tides?
Tides are vertical changes in the level of the sea. As the sea level reaches its highest point in a given location, a high tide is achieved. When the sea level reaches its lowest point in a given location, a low tide occurs. The difference between the sea's level at high and low tides is defined as the tidal range. A tidal day lasts 24 hours and 50 minutes and normally contains two high tides and two low tides. Accordingly, high and low tides arrive 50 minutes later each day.
Creation of Tides
Four factors combine to create and affect the tides. The sun is a massive body which exerts a gravitational pull on the earth, including the seas. The moon exerts a gravitational pull on the seas that is 2.25 times stronger than the sun. The spin of the Earth creates centrifugal forces that move the seas. The local topography can affect the flow of tides.
Neap Tides
Neap tides represent the smallest tidal range. This occurs when the sun and moon are at right angles, partially cancelling out their gravitational pull. This reduction in gravitational pull results in a tidal range that is 10 to 30 percent less than the mean tidal range. This occurs twice during the 28-day lunar cycle, when the moon is one-quarter full.
Spring Tides
Spring tides occur when the Earth, moon and sun are in syzygy and represent the largest tidal range. Syzygy is an astronomical term used to describe the alignment of three or more celestial bodies in a straight line. It is commonly used to refer to the alignment of the Earth, sun and moon. When this occurs, the gravitational pull of the moon and sun are combined, producing the largest tidal ranges. As with the neap tide, spring tides occur twice during a lunar cycle, but at the full and dark moon.
Other Factors
The oceans have tremendous mass and are viscous in nature. This produces both momentum and friction. Neap and spring tides occur seven days apart. However, as a result of these forces, the arrival of neap and spring tides occurs approximately a day behind the actual lunar calendar.
Local topography can have a huge impact on local tides. For instance, Morecambe Bay in Lancashire is known for its extreme tidal changes caused by the topography of the bay. Over the years, many individuals, unaware of these dangers, have lost their lives. Venturing out during low tides, they are overtaken by the powerful, fast-moving, incoming high tides and drown. This danger reaches its peak during spring tides. The Bay of Fundy, which has some of the highest tides on record, is another example of the effects of local topography on tidal levels.