Spring and Neap Tides

We sometimes hear on the news during a coastal storm that the effects on the shoreline will be more severe due to especially high tides. This is due to the position of the moon and the sun in relation to the Earth and the resulting gravitational attraction at various stages in the lunar cycle.

When the moon is in a position relative to the Earth where it is pulling in a direction that is in the same axis as the sun (new and full moon), both bodies are exerting a force on the Earth that is cumulative. This is called conjunction, and because these forces are both working to pull the Earth's surface in the same directional axis, we experience higher high tides and lower low tides than we see throughout the month at a given location. This is the period during the lunar cycle that we experiencespring tides (Figure 2.13).

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Figure 2.13: Figure showing the positions of the sun, moon, and Earth during opposition. Spring tides occur when the sun and moon are aligned causing higher high tides and lower low tides during full and new moon.
Credit: S. Wilde

When the moon is in a position that is at right angles to the sun, the gravitational pull of the two bodies are working in opposing directions on the Earth's surface. This is called opposition, and because these forces are working against each other, we experience moderate changes in water levels. We refer to this period in the lunar cycle as neap tides which occur during quarter moons. (Figure 2.14)

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Figure 2.14: Figure showing the positions of the sun, moon, and Earth during quadrature. Neap tides occur when the sun and moon are acting on the earth in opposing direction.
Credit: S. Wilde

The difference during spring and neap tides is evident in the tide graphs for a tidal gauge in Halifax Harbor shown in Figure 2.13 (Spring tide) and Figure 2.14 (Neap tide). This tide gauge is measuring water levels at the same location, however during a spring tide the high water is almost 6.5ft while during the neap tide, the high water is approximately 4.8ft. The low tide during the spring tide is 0.2ft and during the neap tide is 1.5ft. The highs and lows during the full or new moon are much more pronounced than they are during the quarter moons. That is why a storm event paired with a spring tide may cause more serious damage along a coastline than a storm event paired with a neap tide.

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Figure 2.15: Tidal graph for Halifax Harbor in Nova Scotia during a spring tide. Note the lunar phase and the high and low water readings (horizontal red bars) compared to those presented in Figure 2.16.
Credit: S. Wilde

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Figure 2.16: Tidal graph for Halifax Harbor during a neap tide. Note the lunar phase and the high and low water readings (horizontal red bars) compared to those presented in Figure 2.15.
Credit: S. Wilde