Spring tides and neap tides: why the sea breathes on a 14-day cycle

Guide · Tides Pro

Go back to the same beach a week later and the sea looks like a different animal. Seven days ago low tide uncovered a huge tongue of sand and rock; today it barely leaves a damp strip below the promenade. That is no accident: it is the rhythm of spring tides and neap tides, the fortnightly pulse the ocean keeps in answer to the sun and the moon. Once you understand it, you plan a day on the coast, a fishing session or a foraging trip completely differently.

What spring tides and neap tides are

Spring tides — nothing to do with the season — are the days when the tide has the most travel: high tides climb higher and low tides drop lower than usual. Neap tides are the opposite: the sea rises and falls half-heartedly, and the difference between high and low water stays small.

On the Iberian coast this rhythm is usually summed up in one number: the tidal coefficient, a 0–120 scale computed with the formula of the Spanish hydrographic office (IHM). Below 40 you are in neaps; from 40 to 79, mid tides; from 80 up, springs. It is the single most useful number to check before choosing your day on the water.

The physics: sun and moon pulling on the same rope

The moon is the main driver of the tides, but the sun pulls on the ocean too. When the two line up with the Earth — at new moon and full moon, what astronomers call syzygy — their pulls add together and the tide grows: that is where springs are born.

At first and last quarter, by contrast, sun and moon sit at right angles to each other as seen from Earth (quadrature) and their effects partly cancel out. The result is neaps.

One detail that surprises a lot of people: springs happen at full moon and at new moon alike. The moon does not need to be shining for its tide to be at maximum.

The 14-day cycle and the "age of the tide"

Every lunar month brings two syzygies and two quadratures, so the sea alternates springs and neaps roughly every 14 days. If this week's low tides look timid, make a note: in a week's time the sea will uncover what it is hiding today.

There is a fine detail that marks out people who really know their tides: the ocean does not respond instantly. Because of its inertia, spring tides arrive 1 to 2 days after the new or full moon — a lag known as the age of the tide. Head out on the day of the full moon itself to "catch the spring" and you are early.

Remember too that the semidiurnal cycle runs about 12 h 25 min, which is why each day's high and low waters arrive almost an hour later than the day before.

What changes in the water (and on the shore)

• The range. On springs, high tide reaches places it almost never touches and low tide uncovers rocks, pools and sandbanks that spend weeks underwater.
• The currents. The sea has to move far more water in the same time, so flood and ebb currents become noticeably stronger — especially in estuaries, harbour mouths and channels.
• The speed of change. At the height of a spring tide the waterline advances or retreats visibly; what was a dry walk turns into water in a matter of minutes.
• On neaps, calm. Little range, gentle currents and a sea that barely shifts all day.

Springs or neaps? It depends on the plan

Springs rule the intertidal zone: low water exposes "fresh" rock, making them the big days for shellfish foraging and exploring rock pools. They also tend to fish well from the shore: moving water stirs up food and switches predators on, especially during the hours of strongest current.

Neaps are the friend of the calm swim, snorkelling and paddleboarding: the sea stays put, currents ease off and the beach hardly changes all afternoon. If you are out with kids or still learning, these are your days.

Springs or neaps, the sea is in charge: always check official sources before heading out, learn about the currents in your area, and never turn your back to the sea.

How to plan them on a calendar

The home-made trick is to take a lunar calendar and add one or two days to every new and full moon. The proper method is to read each day's tidal coefficient and choose with it: a 95 and a 38 are two entirely different plans.

Tides Pro does the work for you: it computes the tide for 90 ports with the FES2022b harmonic model — median accuracy of ±10 minutes on the open coast, validated against the IHM — and flags each day's coefficient so you can see at a glance when the next springs are due.

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