King tides

What is a king tide?

King tide is the name people give to the highest predicted tides of the year. NOAA calls it “a popular, non-scientific term people often use to describe exceptionally high tides”, and that is exactly what it is: a word for something the tide tables already contain, not a category an oceanographer would use.

The thing itself has a duller name — a perigean spring tide. It happens when a spring tide, the big tide that comes round at new and full Moon, lands at about the same time as perigee, the point in the month where the Moon is closest to the Earth. Nothing unusual has happened; two ordinary cycles have simply peaked together, as they do a handful of times a year.

The phrase itself spread from Australia and New Zealand, where community photography projects ask people to go out on king tide days and photograph the water at its highest — a picture of where an ordinary tide may reach as sea level rises. That is the reason the term travels so much further than the astronomy behind it.

What causes a king tide?

Two separate cycles have to line up. The first is the phase of the Moon, which decides whether the solar and lunar tides add or fight.

At new and full Moon the Sun, the Earth and the Moon are roughly in line and their tides add: that is a spring tide, and it happens twice in every lunar month whether or not anyone calls it a king tide. On its own it is not enough.

The second cycle is distance. The Moon’s orbit is an ellipse — 384,400 km on average, as close as about 356,000 km at perigee and as far as about 406,700 km at apogee — and tide-raising force falls off as the cube of the distance, not the square. A perigee Moon therefore raises a tide roughly a fifth larger than an average one, while an apogee Moon raises one about a sixth smaller. Perigee comes round about every 28 days, and the phases repeat every 29.53 days, so the two slide slowly in and out of step through the year.

When they coincide you get a perigean spring tide, which NOAA says happens often between six and eight times a year. In its own words those tides run “more than a foot higher than high tides during apogean spring tides”, and three feet or more in a place like Anchorage, where the range is enormous to start with. A foot does not sound like much until it is a foot above the top of a sea wall.

King tide dates for this year

Because it is astronomy, the dates are settled years ahead and no forecast is involved. This site marks a spring peak as a king tide when its tidal coefficient — our single number for the size of the tide worldwide that day — reaches 100, a threshold that gave six to eight days in the years we checked — the same count NOAA gives for perigean spring tides.

A king tide date is the day the coefficient peaks, sampled at noon UTC. The water at your own coast peaks one to two days later — the age of the tide, the time an ocean basin takes to answer — so treat the date as the middle of a three-day window rather than an appointment. On a coast with two high waters a day, both of them will be big; your station’s own table says which is the bigger, and by how much.

Why the biggest king tides cluster near the equinoxes

There is a third cycle underneath the other two, and it belongs to the Sun. Around the equinoxes the Sun stands over the equator, so its declination is near zero and its tide is symmetrical about the equator — which puts as much of its force as it ever puts into the twice-daily tide most coasts actually run on. At the solstices, with the Sun far north or south, more of the same force goes into a once-daily component that most coasts barely feel.

The practical effect is that the largest tides of a year are often within a month or two of an equinox. It is a tendency, not a rule: perigee drifts through the calendar on its own 28-day schedule, so in any given year the biggest peak may land nowhere near either equinox. That is why we publish the dates rather than a season.

King tides and flooding

On a low-lying coast a king tide alone can produce what NOAA calls minor high tide flooding, or nuisance flooding: water across a road at the top of the tide, a car park under a few centimetres, storm drains running backwards for an hour. It drains as the tide ebbs, which is what separates it from the other kind.

Major flooding needs weather on top. NOAA is specific that it takes strong onshore winds and the barometric pressure changes of a coastal storm — and neither of those is in any tide prediction, ours included. Every height on this site is astronomy only. Low pressure lets the sea stand roughly a centimetre higher for each hectopascal it drops, and a storm can move the level by half a metre or more, in either direction. A king tide is dangerous mainly because it uses up the margin that would otherwise absorb that.

One regional caution: on the Pacific coast of North America the memorable king tide flooding events fall in autumn and winter, which is a local coincidence of the astronomy with a seasonally higher sea level and the storm season, not something the Moon does everywhere. Somewhere else, the same coefficient falls in a calm month and nobody notices it.

King tides where you are

Open any station page and the month’s coefficients are drawn as a strip of bars, a bar a day, in four colours. The top band — 90 and over, our “very high” — covers the four or five days around each fortnight’s peak; from 100 up we call it a king tide. That is the fastest way to see whether the day you are planning is one of the big ones (how we compute it).

The coefficient is the same number everywhere in the world on a given day, to within a point or two between time zones, and that is the point at which you have to stop reading it and open a station. A king tide at Depoe Bay on the Oregon coast, at San Francisco inside the Golden Gate and at Key West is the same astronomical event and three completely different numbers of metres, because the size of a tide is set by the shape of the coast that receives it (see tidal range).

Read the low water as well as the high. The same alignment that pushes high water up pulls low water down, so a king tide day is also the day a channel empties, a mooring dries out and a sandbar you have never seen appears. Our heights are measured from mean sea level, so those lows print as negative numbers — that is the datum talking, not a mistake (what is chart datum?).

King tide, spring tide, storm surge: the difference

Three words that get used for one another, and only two of them are tides:

The sizes are as different as the causes. The step from an ordinary spring to a king tide is NOAA’s foot or so, more where the range is already large. A surge is worth half a metre or more and answers to nobody’s calendar. Coastal flooding is almost always the third landing on top of the second.

Questions people ask