King tide is the popular name for an especially high tide. The phrase is used mostly in Australia, New Zealand and other Pacific nations.
In the lunar month, the highest tides occur roughly every 14 days, at the new and full moons, when the gravitational pull of the moon and the sun are in line. These are called spring tides.
The earth moves around the sun in an elliptical orbit that takes a little over 365 days to complete. The sun has an influence on the tides. Its gravitational force is greatest when the earth is closest to the sun (perihelion – early January) and least when the sun is farthest from earth (aphelion – early July).
The moon has a larger effect on the tides than the sun. The moon moves around the earth in an elliptical orbit that takes about 29 days to complete. The gravitational force is greatest when the moon is closest to the earth (perigee) and least when it is farthest from the earth (apogee – about two weeks after perigee).
The king tides occur when the Earth, moon and sun are aligned at perigee and perihelion. On Australia's East Coast, the highest tides occur during the winter months of June, July and August, and the summer months of December, January and February. The highest of each of these periods (i.e., one in winter and one in summer, totaling two per year) are known as the king tides.
The winter king tide usually occurs at night and therefore goes unnoticed. Consequently the summer king tide usually catches the most attention. On such days the surf is particularly dangerous — tall waves, long clean breaks, strong rips, consistent sets: ideal for surfing, but seductive and dangerous for inexperienced swimmers.
King tides are particularly formidable on the island country of Tuvalu, where they can devastate the low lying atoll's people and wildlife. Sea level rise may exacerbate these effects.[1][2]
As with all spring tides, king tides occur with the coming of new and full moons. Some traditional mythologies hold that such times cause madness, a possible origin of the word "lunacy".[3]