What Is a Spring Tide? Twice a Month, Even in Winter

Spring tides do not wait for spring. The one under the North Sea flood of 1953 arrived on the night of 31 January, when a storm tide swept across the North Sea and flooded the low-lying coasts around it, killing 1,836 people in the Netherlands and 307 on land in the United Kingdom.

The water peaked at night and caught many people asleep, Herman Gerritsen of Delft Hydraulics wrote in a 2005 retrospective for the Royal Society. The storm, his paper notes, “occurred in combination with spring tide.”

Two nights earlier the Moon had been full: NASA’s table of moon phases lists a total lunar eclipse at the full moon of 29 January, 23:44 Universal Time, which means the Sun, Earth and Moon were almost exactly in line. A spring tide in January breaks no rule, because the tide’s name has nothing to do with the season.

What is a spring tide?

A spring tide is the larger-than-average tide that comes twice a month, around every new moon and full moon, when the Sun, Earth and Moon line up and the Sun’s tidal pull adds to the Moon’s. High tides run higher and low tides lower than average. It happens in every season, and the name has nothing to do with spring.

NOAA’s National Ocean Service says spring tides “occur twice each lunar month all year long, without regard to the season.” A lunar month averages about 29.5 days, so one arrives roughly every two weeks.

The Sun pulls harder but raises a tide half the Moon’s

The Sun has about 27 million times the Moon’s mass, according to NOAA’s tides tutorial.

“Based on its mass, the sun’s gravitational attraction to the Earth is more than 177 times greater than that of the moon to the Earth,” NOAA’s tides tutorial says.

A tide comes from the difference in pull between the side of Earth nearest the Moon or Sun and the side farthest away. That difference shrinks with the cube of distance, while gravity itself shrinks only with the square. The same difference in pull, from Earth, once flexed the young Moon’s rock until its spin slowed to one turn per orbit.

The Sun is 390 times farther away than the Moon, and 390 cubed is about 59 million. Divide the Sun’s 27-million-fold mass advantage by that and roughly half is left. The Sun’s tide-generating force comes out at “about half that of the moon,” NOAA says, and the UK’s National Tidal and Sea Level Facility (NTSLF) puts it at 0.46 of the Moon’s.

Neap tides fall a week later, at the half moon

About seven days after a spring tide, the Moon reaches first or third quarter and sits at a right angle to the Sun. The solar tide then “partially cancels out the lunar tide,” NOAA’s page on tidal variations explains, leaving a neap tide, with high tides a little lower and low tides a little higher than average.

	Spring tide	Neap tide
Moon phase	New moon and full moon	First and third quarter
Sun, Earth and Moon	In a line	At a right angle
Solar tide	Adds to the lunar tide	Partly cancels it
Tidal range	Largest of the month	Smallest of the month
Peak timing	1-2 days after new or full moon	About 7 days after a spring tide

The biggest tide arrives a day or two late

The shape of the real oceans and their friction hold back the peak, so the biggest tides arrive one to two days after new or full moon, NTSLF explains. NOAA’s tide glossary calls the delay the age of phase inequality, or the age of tide. The 1953 storm struck about two days after its full moon.

Pliny the Elder, the Roman encyclopedist who dismissed the cinnamon-bird stories as a pricing trick, had recorded the delay by the first century AD. The biggest tides came “not exactly at the full nor at the new moon, but after them,” his Natural History says, in Bostock and Riley’s 1855 translation.

Nobody is sure where the name came from

The popular reading, a tide that belongs to springtime, is a myth, since spring tides come every month. The real origin is unsettled.

NOAA says the term comes from the tide “springing forth,” and Parks Canada’s Fundy National Park derives it from the Old English “springan,” which it says “means to well up.” NTSLF is more cautious: the origins “are not known for certain,” and its candidates are “the Scandinavian word for ‘leap up’” and “a place where water wells up from the ground.”

Etymonline supplies the root, Old English springan, “to leap, leap up, jump,” also said of a fountain that would “burst forth.” In Middle English the noun spring was used of sunrise, the waxing of the moon and “rising tides.”

The spelling springtide did double duty, meaning springtime in the 1520s and the high tide from the 1540s.

Equinox or solstice, depending on the coast

On coasts with two roughly equal tides a day, like Britain’s, they are “particularly large close to the equinoxes (21 March and 21 September) when the sun is overhead at the equator,” NTSLF says. Pliny noticed this as well: tides rise “higher at the equinoxes, and more so at the autumnal than at the vernal.”

Coasts with one tide a day, like many in the Gulf of Mexico, barely register the spring and neap cycle; for that kind of tide NOAA’s glossary calls the spring range “of no practical significance.” The biggest ranges there follow the Moon’s swing north and south of the equator, peaking when lunar declination is greatest, NTSLF says.

Those coasts, and mixed-tide coasts like the US Pacific, see their biggest tides nearer the solstices. NOAA’s glossary says the tropic range “may be expected to be especially large” then, and California’s Coastal Commission says its king tides, “the highest high tides of the year,” come in winter, around the New Year.

Perigee adds a foot, and a popular name

Often between 6 and 8 times a year, a new or full moon also coincides closely with perigee, the Moon’s closest approach to Earth, producing what NOAA calls a perigean spring tide. On coasts with big tides, high tides can then run more than a foot above those of spring tides near apogee. In Anchorage, Alaska, which NOAA says has a tidal range over 30 feet, the gap can reach 3 feet or more at high tide. People often call such extremes king tides, a “non-scientific term” in NOAA’s words.

Tides “can be predicted far in advance and with a high degree of accuracy,” NTSLF notes, while storm surges can be forecast only about two to five days ahead. The spring tide under the 1953 flood could be read off a tide table long before the storm formed, two nights after a full moon in the middle of winter. Strictly FYI.