Beachgoer's Guide to Tides

A Beachgoer’s Guide to Tides

Skim this to know more about tidal effects than most fishermen Some ancient myth-makers held that the earth’s pulse or breathing caused the tides. The Greeks began to notice the moon’s influence when they began venturing out of the relatively tideless Mediterranean. Our modern understanding of tides is based on Sir Isaac Newton’s equilibrium theory of tides, which described the gravitational attractions of the sun and moon on the earth’s waters.

Today, tides can be predicted with astronomical precision and need no longer be a mystery. Here’s what landlubbers will fin when they are at the beach.

If you live on the East Coast, expect the tides to be semidiurnal: That’s when high and low tides occur twice per lunar day, and the heights of both the first and second set of tides are roughly the same.

A lunar day is 50 minutes longer than a day on Planet Earth, which is why in many places high and low tides occur about 50 minutes later than the corresponding tides of the previous day.

If you live on the West Coast, expect mixed tides: The tides on America’s left coast rise and fall twice per lunar day, but the heights or the second set of tides differ from the first. The different tides each day are termed higher high water, lower high water, higher low water, and lower low water. Their order of occurrence varies over the course of the moon and from place to place.

Along the Gulf of Mexico, the tides are diurnal: Here, high and low tides appear only once every day. About twice a month, near the time of the new and full moon, the tidal range between high and low tides is usually 20 percent above average. These tides, known as spring tides, occur when the sun and moon are in a straight line with the earth. When the sun and moon are at right angles to each other with respect to the earth, tidal ranges between high and low tides are about 20 percent less than average. These tides, known as neap tides, occur around the time of the first and third quarters of the moon.

For another big swing in height between high and low tide, wait until the moon is at perigee: That’s when the moon is at its closest point to the earth each month and is when the tidal range between high and low tide is greatest. Roughly two weeks later the moon is at apogee, which is its farthest point from the earth for the month. That’s when the moon’s influence is at a minimum. During the course of the month, daily inequalities between successive high or low tided also can occur: This happens as the moon moves from about 28 degrees north of the equator to 28 degrees south. When the moon is at one of these extremes, the difference in height between morning and evening tides is greatest. When the moon is at the equator, tides are roughly equal.

The tidal range also increases in January: That’s when the earth’s elliptical orbit around the sun brings the planet closest to the sun, in what astronomers call the state of perihelion. In July, when the earth is farthest away from the sun in what astronomers refer to as the state of aphelion, the tidal range decreases. The shape and depth of ocean basins change tidal ranges, too: The smallest differences in the heights between high and low tides occur along ocean coasts. For example, spring tidal ranges (near the time of the new and full moon) vary from about 2 feet on the Gulf Coast to as much as 8 or 9 feet on the California Coast.

The largest tidal ranges are found in tidal inlets, estuaries, and salt marshes. The Bay of Fundy in Canada has the greatest difference in the world between high and low tide. There, the funnel-like opening of the bay concentrates the energy and increases the height of the incoming tide, resulting in average tidal ranges of about 45 feet. Meteorological conditions can also disrupt normal tide patterns: While tide tables provide accurate times and heights of high and low tide, strong and persistent winds and the low atmospheric pressure associated with storm systems can alter the time and height of high or low tide.

If you are planning to go clamming or to collect seashells at low tide and there is a strong onshore breeze, you may want to delay for as long as an hour after the predicted time of low tide. Horizontal water movements, known as tidal currents, are generally strongest midway between high and low tide: When the tide rises and water flows in to fill estuaries and inlets, the water is called a flood current. When the tide goes out and water drains from these coastal areas, the flow is called an ebb current. Slack currents are found around the time of both high and low tide.

Surf fishing is best done when tidal currents are strongest: Strong flood currents force bait fish up closer to the beach and cause from to school up tighter and hide behind features such as rocks or jetties. Game fish such as bluefish or striped bass take advantage of these conditions, and so should knowledgeable surf fishermen, advises marine fisheries biologist Gregg Skomal. Game fish lie in wait for the bait fish to be swept in and out of inlets, estuaries, and bays by flood and ebb currents. Slack water is usually the worst time to fish. Boaters shouldn’t rely on the tidal charts in coastal newspapers. While most coastal newspapers list the times and heights of high and low tide, boaters need more tidal information than the newspapers provide in order to ensurer safe passage over the ocean floor and to know here to anchor. They should consult the tide and tidal current tables published each year by the government.

The Moon and Tides

The tide is the rise and fall of water throughout the earth’s oceans. Every 12 hours and 26 minutes, tides are created primarily by the moon’s pull of gravity on the water. When the earth is nearer the moon, water is pulled toward the moon, producing a high tide. This happens on the opposite side of the earth at the same time. Spring tides occur when the sun and the moon are directly in line. The moon is either in front of or behind the earth. This produces a very high tide twice a month. Neap tides do not rise as high as normal because the moon is at right angles to the sun. A neap tide occurs twice a month.

Safety for Swimmers and Surfers

Lifeguards recommend swimming off sheltered beaches such as those found in coves or behind a point or peninsula. Swimming is easiest out beyond the breakwaters and away from where the biggest sets of waves are breaking. Eyeball how longshore currents are flowing by watching floating debris or swimmers, and swim with the current to limit fatigue and frustration.

Many years ago Karl Tallman, a lifeguard for the California Department of Parks and Recreation gave these additional tips:

• Spilling breakers provide body or board surfers with the longest ride. Such waves commonly occur on relatively flat beaches and are characterized by foam and bubbles that spill down the front of the wave.
• Plunging breakers can break with great force right on top of an unsuspecting surf swimmer. They are found on moderately steep beaches. The crests of these waves curl over a pocket of air and result in splash-up. As waves, they are short-lived and are not the best for surfing.
• Surging breakers slide up and down the beach creating very low challenges for the surf swimmer. They occur on steep beaches and produce little or no bubbles.
• Rip currents can endanger even the most experienced swimmer. Remember to swim parallel to the shore to escape the current or allow it to carry you out to where its strength diminishes.

Expert choice: To obtain the tide and tidal current tables for your coastal region, contact National Oceanic and Atmospheric Administration Distribution Branch.