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| What are Spring Tides? What are Neap Tides?
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February 5, 2002
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Dan Jonson isn't afraid to admit he doesn't understand something. He sent this inquiry:
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R obert Hale was kind enough to explain Spring and Neap tides in an e-mail to me last year. That e-mail was lost and never printed. I was wondering if he could describe them to me again.
Thanks!
Dan Jonson
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Response
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Spring tides are large tides. They occur twice a month, at full moon and new moon. Neap tides are small tides. They occur twice a month, at half-moon. In most parts of the Northwest we have four tides a day, two larger tides and two smaller tides. During the time of neap tides the larger daily swings will be smaller than at full moon or new moon, and the smaller tides may be reduced to little or no change at all.
During springs, the large daily tides will be very large, and even the small daily tides can be significant.
When the sun and moon align just right, the spring tidal range can be extreme. This is what makes some winter storms so damaging. An unusually large spring tide combines with heavy river runoff, high winds, and low barometric pressure to push the high water over beachfront bulkheads and across roads.
The tidal range changes each day as we progress from new moon (spring), to half-moon (neap), to full moon (spring). It takes only a week to go from neap to spring. Thus a narrow pass that was easily run at maximum current a week ago may be a dangerous tidal rapids today. Watch the moon (study your tide and current tables, too).
--Bob Hale
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