What's a minus tide?

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by Jeff Renner

Bio | Email | Follow: @Jeffrennerwx

KING5.com

Posted on May 21, 2009 at 6:33 PM

Updated Thursday, Jan 14 at 9:20 AM

What's a minus tide?

A minus tide, also sometimes called a negative tide, is an unusually low tide. Tide 'heights' are referenced to a base level set as zero. This level, called the 'datum', is the same level used to reference water depths on NOAA navigational charts.

Usually, low tides still remain a bit above zero, often a foot or two above. But the swing from high tide to low tide levels is greatest when the sun and moon are either on exact opposite sides of the earth, or especially when they're on the same side of the earth. This corresponds to a full or a 'new' moon. The gravitational 'tug' by both the sun and moon pull water toward them. As the earth rotates under the oceans, high tides occur when that part of the earth is facing either the moon or sun (or both). Low tides occur about six hours later, when the earth has turned ninety degrees away from either of those 'bulges' of water.

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