Spring and neap tides, and the tide coefficient

Spring tides and neap tides

Tides aren’t the same size every day. Over about a week they build to a peak, the spring tides, then shrink to a low point, the neap tides, and the cycle starts again.

Despite the name, spring tides have nothing to do with the season. The word comes from the idea of the tide “springing forth,” and they happen year-round. Neap is often said to come from an old English word for scant or lacking.

Why the moon phase sets the size of the tide

Both the moon and the sun stretch the ocean into bulges, and the moon’s effect is roughly twice the sun’s. What matters is how the two line up:

• At new moon and full moon, the sun, Earth and moon are nearly in a line. Their effects add up: high tides run higher and low tides run lower.
• At first quarter and last quarter, the sun and moon pull at right angles. Their effects partly cancel, and the tide is smaller.

At new moon you can’t see the moon, but it’s there, sitting between the Earth and the sun and pulling in the same line as the sun.

The ocean takes a while to respond, so spring tides usually peak a day or two after the new or full moon, and neap tides a day or two after the quarter moons. There are two “spring tide” phases in every lunar month of 29.5 days, so the full cycle takes just under 15 days. For the physics behind the bulges, see what causes tides.

How much difference does it make?

It depends on the place, but the change is easy to notice. Take a harbor whose average range (the difference between high and low tide) is 6 feet. On spring tides it might see 7 feet or more, with a low tide below zero on the chart. On neap tides it might see only 4 to 5 feet, and the water seems to barely move.

That matters more than it sounds. The tide still has about six hours to rise, so a bigger range means the water moves faster: more inches per hour on the beach, and stronger currents in inlets and channels. In US tide charts, spring tides are when you see the minus tides, the lows that drop below the chart’s zero. We explain those in how to read a tide chart.

King tides and perigean spring tides

Not all spring tides are equal. Two things make some of them bigger:

• The moon’s distance. Its orbit isn’t a circle. At perigee, its closest point to the Earth, its pull is stronger; at apogee, the farthest point, weaker. Six to eight times a year, a new or full moon lines up closely with perigee, producing a perigean spring tide. According to NOAA, high tides during a perigean spring tide are often more than a foot higher than during an apogean spring tide (when the moon is farthest away), and 3 feet or more in places like Anchorage, Alaska.
• The season. Near the March and September equinoxes, the sun sits over the equator, which boosts spring tides on coasts with two tides a day. In some places, seasonal changes in sea level add a bit more at certain times of year.

King tide isn’t a scientific term. It’s the name many coastal communities use for the highest tides of the year, usually perigean spring tides. The extra water may not sound like much, but in low-lying neighborhoods of Charleston, Miami or Norfolk, it can be the difference between a dry street and a flooded one, even on a sunny day.

The tide coefficient: how Europe rates tides

In France and Spain, tide tables add a number that Americans rarely see: the tide coefficient. It goes back to the work of the French mathematician Pierre-Simon Laplace, and today it’s calculated by SHOM, the French hydrographic service, using the port of Brest in Brittany as its reference.

The coefficient compares the range of each day’s tide with a reference spring tide at Brest and puts it on a scale from 20 to 120. It has no units: it’s not a height in feet or meters, but a rating of how big the tide is.

Coefficient	What it means
20	The smallest possible neap tide
45	An average neap tide
70	The line between neap and spring tides
95	An average spring tide
100	The average spring tide around the equinoxes
120	The largest possible tide

The key point is that the coefficient is proportional to the range. If your harbor has a 13-foot range on a coefficient 95 tide, a coefficient 45 tide will give it about 13 × 45 ÷ 95, or roughly 6 feet. Same six hours, less than half the water moving.

The catch is that the coefficient doesn’t tell you how much the water rises where you are. A coefficient 100 tide means a range of several meters on Spain’s north coast and less than a foot and a half in Barcelona. So you still need the heights from a local tide table.

US tide tables don’t publish a coefficient. If you want the same information, look at the predicted range for the day or simply check the moon phase. And if you travel to Brittany, Normandy or northern Spain, now you know what that number next to the tide times means.

What spring and neap tides mean on the water

• Fishing. Stronger current on spring tides moves bait and often gets fish feeding. Very big tides can muddy the water and make it hard to hold bottom, so many anglers like the days just before or after the peak. More in moon phases and fishing.
• Clamming and tide pools. The lowest lows of the month come with spring tides. In the Pacific Northwest, clam digs and tide pool walks are planned around them.
• Boating. Spring tides mean faster currents in inlets and less water over bars at low tide. Neap tides are gentler for anchoring and for beginners.
• Coastal flooding. A spring or king tide plus onshore wind or a storm is the combination that overtops seawalls and floods coastal roads.

How to know when the big tides are coming

1. Find the next new or full moon. Expect the biggest tides of the two-week cycle a day or two later.
2. Check whether the moon is near perigee that week. If it is, the tides will run a little bigger than usual.
3. If the date falls near an equinox, or your town is in its king tide season, expect some of the highest water of the year.

Then check the tide table for your exact spot to get the times and heights for that day.