King tides are the year's highest tides — astronomy, not weather. How perigee, syzygy and the solstices stack up, and why they preview our coastal future.
A couple of times a year, usually on a calm day with no storm in sight, the ocean simply shows up where it isn't supposed to: across the beach car park, over the footpath, into the lowest street. The sky is blue, the wind light, yet the water keeps rising past every familiar mark. That's a king tide — the informal name for the highest tides of the year — and understanding it takes about ten minutes of astronomy and a little humility about how much coastlines are about to change.
King tides are not storms, and they are not caused by sea-level rise. They are the predictable, astronomical extreme of the ordinary tide cycle, occurring when several rhythms of the Earth–Moon–Sun system line up. What makes them newly famous is that they offer a free preview: today's king tide is a decent estimate of an ordinary high tide a few decades from now.
Every king tide is built from the same ingredients as every other high tide, so it's worth a two-minute refresher. The Moon and the Sun both raise tides — the Moon through gravity's gradient across the Earth, the Sun at about 46 percent of the Moon's strength. Twice a month, at new moon and full moon, Sun, Earth and Moon fall roughly into a straight line. Astronomers call that alignment syzygy. At syzygy the solar and lunar tides stack: high tides run higher, low tides run lower. These fortnightly extremes are spring tides — "springing forth", not the season.
So every coast already sees amplified tides twice a month. A king tide is what happens when syzygy recruits three more allies.
The Moon's orbit is not a circle but a moderately squashed ellipse. Its distance from Earth swings by roughly 50,000 kilometres over a month — from about 356,500 km at closest approach (perigee) to about 406,500 km at furthest (apogee).
Tidal force falls off steeply with distance — with the cube of it, in fact. Because the numbers work out that way, a perigee pass can strengthen the Moon's tidal pull by around 30 percent compared with apogee. The ocean notices. Tides near perigee run higher than tides near apogee, all else equal.
Perigee swings around on a 27.5-day cycle (the anomalistic month), while spring tides follow the 29.5-day lunar cycle. Because the two cycles are different lengths, they drift in and out of phase — which is exactly what sets up the show.
When perigee happens to fall within a day or two of a new or full moon, the two amplification mechanisms stack. The alignment is called a perigean spring tide, and this is the formal, scientific name for what most people call a king tide. "King tide" itself has no official definition — it started as slang (popularised in Australia and the Pacific) and stuck because it's honest: it names the ruling tide of the year.
There's one more dial. The Sun's apparent path through the sky matters too: in many mid-latitude regions the very largest tides occur in the months around the solstices, when the Sun or the Moon sits far north or south of the equator and the geometry of the tide-raising forces works out most favourably for that hemisphere. That's why your local king tide season falls in particular months — in California, typically around December–January and mid-year; the exact calendar varies with latitude and coastline.
The result: a few windows a year, each a few days long, of the highest high water the astronomy can arrange. Piling it up:
When all three coincide, tide gauges record values that sit near the top of the tide tables for that year. Note that king tides are slightly different everywhere: local basin shape, shelf width and amphidromic geometry decide exactly how big the water actually gets, in the same way basin physics shapes every local tide regime.
On a fine day, a king tide is curious; on a bad-weather day, it is the multiplier. What's physically happening is simple: the tide is the platform the sea stands on. Everything else — storm surge, wave runup, swell set-up — is added on top. Raise the platform 40 centimetres and the same storm throws noticeably further.
Around a king tide you'll typically see some combination of:
Here is why coastal planners have adopted king tides as a public-education tool. Global mean sea level has already risen significantly since 1900 and is projected to keep rising through this century as oceans warm and ice sheets melt. Precisely because king tides are astronomical — independent of that trend — they act as a benchmark: they show, on a repeatable schedule, what a somewhat higher resting sea level would do to a familiar place, with no drama added.
A growing network of community photo-monitoring projects (the best known have run on the US West Coast, Australia and New Zealand for years) asks a simple favour: photograph the shoreline at the king tide high, from the same spot, year after year. The archive becomes a quiet, local way to watch sea level — and a practical record for councils of which pipes, paths and pump stations fail first. If you want to contribute, this is one of the easiest forms of useful coastal citizen science there is.
For planners, the arithmetic is stark: today's king tide is an occasional Tuesday in the decades ahead. The tide isn't changing; the baseline is.
Kelp forests live just below the tideline, so king tides never touch the forest directly — but they touch everything around it.
The dates are published, years in advance, by national tide services and in any good tide-table app — look for the fortnight where predicted highs peak for the year. To observe one usefully: pick a fixed spot (the same set of stairs, the same seawall), arrive a little before the published high, note the still-water level on a fixed landmark, then come back at an ordinary high tide a fortnight later and compare. The visible difference — often tens of centimetres — is the size of the astronomical "reserve" we live with. And if you can, combine the trip with a look at the buoy data for the day; seeing how the water level charts beside the astronomical prediction is a small, satisfying lesson in how the ocean is modelled.
No. King tides are purely astronomical — they would occur on an unchanged-climate Earth every year on schedule. What climate change does is raise the baseline sea level underneath them, so each year's king ride starts from a slightly higher platform. That's the sense in which they're "a glimpse of the future sea".
Typically two to four king-tide windows per year, each lasting a few days around the peaks of the lunar cycle. The exact dates and months vary by latitude and local tidal regime; national tide services publish them years ahead.
Not officially. The formal term is perigean spring tide — a spring tide (at new or full moon) that falls within a day or two of perigee, the Moon's closest monthly approach to Earth. "King tide" began as colloquial usage and is now employed widely by councils, photographers and educators because it's plain.
That's called sunny-day (or nuisance) flooding, and it's almost always a king-tide phenomenon: the extreme high tide alone, riding up storm drains and over low thresholds, with no storm surge required. If a weather system happened to add low pressure, wind and waves on top, the same tide gets much worse.
Two things. Photograph your shoreline at the published king-tide high, from a repeatable location — many regions run community photo projects designed for exactly this. And then return at the corresponding minus low a few days either side of the same dates: the exposed reef flat will be the year's best window into intertidal life that is normally underwater.
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