The year's highest tides

A popular name
for a real thing.

There is no scientific definition of a king tide. There is a very real astronomical event underneath it, and a good reason the water often does not do what the table says.

01 · The word

"King tide" is popular usage, not a technical term

You will not find king tide defined in a tide table, because it is a colloquial Australian and New Zealand expression rather than a term of art. It generally means the highest tides of the year at a particular place, the ones that put water over the boat ramp and up into the park.

The technical term for the ceiling is Highest Astronomical Tide (HAT): the highest level that can be predicted to occur from astronomical causes alone, under average meteorological conditions. HAT is the mirror image of Lowest Astronomical Tide, which is the chart datum Australian tide heights are measured from. More on datums.

A king tide is not a separate kind of tide. It is an ordinary spring tide with several reinforcing factors landing at once.

02 · The mechanism

What actually produces the year's highest predicted tides

Three things stack:

Alignment. A spring tide, when the sun and moon pull in line at new or full moon.
Lunar perigee. The moon's orbit is elliptical, so its distance varies. A spring tide occurring near perigee, when the moon is closest, gives a distinctly larger range. This combination is a perigean spring tide.
The sun's contribution. The earth's distance from the sun and the sun's declination also vary through the year, and their contribution peaks at particular times, which is why the very largest tides cluster in certain months rather than being spread evenly.

When all three line up you get the highest predicted tides of the year. They arrive in clusters of consecutive days rather than as a single date, and they recur on a roughly annual pattern that shifts because none of these cycles is a whole number of days.

One consequence worth stating: because this is astronomy, the dates are known years in advance and appear in the official annual tide tables. Nobody has to guess.

03 · The overlooked half

The winter king tides nobody talks about

Here is a detail most coverage omits. In Queensland, Maritime Safety Queensland points out that the year's highest tides occur in both summer and winter, but the winter ones tend to fall at night.

So the summer ones get photographed, discussed, and called king tides. The winter ones, which can be comparable in height, happen in the dark while everyone is asleep and pass without comment.

This is a good illustration of the general principle that "king tide" describes a social phenomenon as much as a physical one. If your interest is flooding risk rather than photography, the night-time events matter just as much, and they are in the same tables.

04 · Practical

How to find the dates yourself

Atmosphr does not publish tide predictions, so we cannot list the dates for your port. Here is how to get them from the source in a few minutes.

Download the annual tide table for your port from the Bureau of Meteorology tide tables, or from your state authority. Tables are usually published for the current year and the next.
Scan the high water heights for the year and find the largest. Do not stop at the first one; the highest tides come in clusters, and there is usually more than one cluster in the year.
Note the times as well as the dates, and check the table's time basis. A table published in standard time needs an hour added during daylight saving in states that observe it. How to read the table.
Compare against the port's Highest Astronomical Tide if it is published, to see how close the year's peak gets to the astronomical ceiling.

In Queensland, Maritime Safety Queensland publishes a dedicated king tides page, which is the most direct route if you are in that state.

Why we are not listing dates here: tide predictions in Australia are Crown copyright and we do not hold a licence to republish them. Our position on that, in full.

05 · The catch

Why the water is often higher, or lower, than predicted

A king tide prediction is an astronomical calculation. It assumes average weather. Real weather is rarely average, and on the day it can add or subtract a substantial amount.

Barometric pressure. Low atmospheric pressure lets the sea surface rise; high pressure pushes it down. The effect is roughly a centimetre of water for each hectopascal of pressure difference from average, so a deep low can add tens of centimetres on its own.
Wind setup. Sustained onshore wind piles water against the coast. Offshore wind does the reverse and can leave less water than predicted.
Storm surge. The combination of the two under a tropical cyclone or an intense low, which is the dangerous case and can dwarf the astronomical component.
Long-term sea level change. Slow, but it shifts the baseline that all of the above sits on.

So a king tide on a calm, high-pressure day may quietly under-deliver, and an ordinary spring tide under a deep low with onshore wind can flood places a king tide normally would not. The prediction tells you the astronomical part only. The full explanation of prediction versus reality.

06 · Safety

Coastal flooding, and who to listen to

King tides are the days when low-lying coastal roads, car parks, boat ramps and foreshore reserves go under, usually as nuisance flooding rather than as an emergency. When a king tide coincides with a storm, that changes.

For anything with a consequence, follow the official channels rather than a tide table alone:

Bureau of Meteorology warnings, including coastal hazard and flood warnings.
Your state emergency service, and your local council, which usually knows exactly which streets go under and at what height.
For a cyclone, the official storm tide warnings, which combine the astronomical tide with the forecast surge. Never estimate this from a tide table yourself.

Several Australian states run citizen science programs inviting people to photograph king tides at known locations, as a way of showing communities what routine sea levels may look like in future. If you are photographing one, do it from somewhere you can leave.