Spring vs Neap Tides

Tides do not just rise and fall; the size of the rise and fall swells and shrinks on a fortnightly rhythm. The big ones are spring tides and the small ones are neap tides, and both come from where the Moon and Sun sit relative to Earth.

The short version

Spring tides are the larger tides: higher highs and lower lows. They occur around the new moon and the full moon. Neap tides are the smaller tides, with milder highs and lows, and they occur around the first-quarter and third-quarter moons. The word "spring" has nothing to do with the season. It comes from an old sense of the water "springing up".

Why the Moon and Sun matter

Tides are produced mainly by the Moon's gravity, which pulls on the ocean differently on the near side of Earth than on the far side. The Sun also raises tides, but its effect is a bit under half as strong as the Moon's because it is so much farther away. Each body creates its own pair of bulges, and what you see at the coast is the combination.

At new moon and full moon, the Sun, Earth and Moon are in a line. The two sets of bulges overlap and reinforce each other, giving a larger tidal range: a spring tide.
At the quarter moons, the Sun and Moon pull at right angles. Their bulges partly cancel, so the range is smaller: a neap tide.

The cycle at a glance

Moon phase	Sun-Earth-Moon geometry	Tide type	Typical effect
New moon	In line, Moon between Earth and Sun	Spring	Larger range
First quarter	Right angles	Neap	Smaller range
Full moon	In line, Earth between Moon and Sun	Spring	Larger range
Last (third) quarter	Right angles	Neap	Smaller range

A full cycle of lunar phases takes about 29.5 days, so there are two spring tides and two neap tides in that time, roughly 14 to 15 days between one spring tide and the next. In practice the biggest tides usually arrive a day or two after the new or full moon, because the ocean takes time to respond. Tide observers call this delay the "age of the tide".

Perigean spring tides and "king tides"

The Moon's orbit is slightly oval. Once every 27.5 days or so it reaches perigee, its closest point to Earth, and its pull is stronger. When perigee lines up with a new or full moon, the result is a perigean spring tide, one of the largest tides of the year. People often call these "king tides". Combined with a storm or strong onshore wind, they are the days when low-lying streets near the coast flood even without rain, which is why coastal managers watch them closely.

What changes on the ground

Beaches and tidepools. The lowest low tides, including the negative ones, come during spring tides, which is when rocky shelves and sandbars are most exposed. Neap tides leave less shoreline revealed.
Boat ramps and docks. A spring high can cover a slip or a causeway that stays dry at neaps. A spring low can strand a boat that floated comfortably during neaps.
Currents. Larger ranges generally mean stronger tidal currents in channels and inlets, while neap tides are generally gentler.
Flooding risk. Spring highs sit higher on the shore, so the margin against surge and heavy rain is smaller.

Spotting it on TideRef

TideRef's "Last 30 Days" panel plots the daily predicted tide range, which is the difference between the highest high and the lowest low each day. Open it for any station and you will see the range rise and fall in waves about two weeks apart, a direct picture of the spring-neap cycle. The Moon row on the main screen is computed in your browser from the average length of the lunar cycle. It flags "spring tides" when the Moon is within about two days of new or full, and "neap tides" within about two days of a quarter. Because it uses an average cycle rather than a full astronomical calculation, treat the phase as accurate to within roughly half a day. The tide heights themselves come from NOAA.

Not every coast behaves the same

The spring-neap cycle is strong on coasts with large tides and more subtle where the tidal range is small. In some diurnal areas, other lunar effects, such as the Moon's changing distance from the equator, shape the pattern in addition to the phase. Use the station you actually plan to visit and compare its table over a couple of weeks rather than assuming a rule of thumb.

Put it into practice: open the TideRef tide clock, pick a station, and compare what you just read with live NOAA predictions.