The term king tide has no scientific definition – in popular usage it refers to any high tide well above average height. The popular concept is that the king tides are the higher high waters which occur around Christmas time. Equally high tides occur in the winter months but these are at night and not as obvious as those at Christmas which generally occur during the day.
Queensland has a relatively large and variable tidal range, for example:
- 7.14m at Hay Point
- 6.58m at Mackay
- 4.88m at Karumba
- 4.83m at Gladstone
- 3.86m at Thursday Island
- 3.50m at Cairns
- 2.73m at Brisbane
- 2.17m on the Sunshine Coast.
The king tides are no more than the very highest tides that occur at each place. They:
- occur naturally and regularly
- are predicted in the tide book
- are not an everyday occurrence.
In a lunar month the highest tides occur at the time of the new moon and full moon (when the gravitational forces of the sun and moon are in line). These are called spring tides and they occur about every 14 days.
In any 1 year there will be 2 spring tides that are the highest, 1 during summer and 1 during winter. These are referred to as king tides. The king tides occur because of the combined influence of a number of astronomical factors related to the Sun and the Moon (and their alignments) and the gravitational attraction they each have on the water surface of the Earth.
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 furthest 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 furthest from the earth (apogee – about 2 weeks after perigee).
The king tides occur when the earth, moon and sun are aligned at perigee and perihelion. The combined effect of the moon's phase and the varying gravitational forces of the sun and moon result in the highest of the spring tides occurring during the summer months of December, January and February and also in the winter months of June, July and August.
During normal weather conditions, the height of the king tides will be similar from year to year. However in abnormal weather conditions (severe storms or cyclones) the low air pressures and strong winds at these times can elevate the sea level above the expected height.