Spring Tides

Spring Tides
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Fig 5.5 – When the moon, earth and sun are lined up, during a full moon or new moon the gravitational pull is complimentary and we experience spring range tidal cycles.

Don’t think of Spring tides just happening during the Spring months, as they actually occur roughly every 14 days, twice every time the moon orbits the earth at the time of the new moon and the full moon. When the sun and the moon line up with the earth (Fig 5.5), their gravitational pull is complimentary creating a more pronounced tidal bulge around the earth. High tides become higher and low tides become lower.

The average of these spring high tides over the year is known as Mean High Water Springs (MHWS) (Fig 5.6). Likewise the spring low tides are lower than usual and the average level over the year is known as the Mean Low Water Springs (MLWS)

The difference between MHWS and MLWS is known as the Spring Range.

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Fig 5.6 – Spring tides have the largest range. Tide level is measured from Chart Datum. The heights of charted object are measured from Mean High Water Springs.

Remember, we use the MHWS level to measure heights of lighthouses, mountains, churches and any other charted object apart from clearances under bridges, overhead lines, pylons etc. As we shall see below these are measured from a high water level known as Highest Astronomical Tide (HAT) which is a slightly higher level than MHWS.