Frequently asked questions
Q. What causes "King Tides"?
The term "king tide" has no scientific definition although 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 are not so apparent as those at Christmas which generally occur during the daylight hours. The highest of these high tides are referred to as "king tides" - one during summer and one during winter.
The "king tides" occur because of the combined influence of a number astronomical factors which are related to the sun and the moon (and their alignments), and the gravitational attraction they each have on the water surface of the earth. See Astronomical tide in the Glossary.
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 each year) and least when the sun is furthest from earth (aphelion early July each year). The moon has a larger effect on the tides than does the sun.
The moon moves around the earth in its elliptical orbit that takes about 29 days to complete. In a lunar month the highest tides occur at the time of the new moon and full moon (when the gravitational forces of sun and moon are in line). These are called "spring" tides and they occur about every 14 days. Because of the elliptical orbit, the distance between the moon and earth changes. The gravitational force is greatest when the moon is closest to earth (perigee) and least when it is furthest from the earth (apogee) about two weeks after perigee.
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.
The situation at mid summer of 1998-99 is a good example:
| The Sun |
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| The Moon |
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| Relative Position of the Sun, Moon and Earth |
At 1300 Hours 2 January 1999: |
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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.
Further information on "king tides" can be found at the Queensland Transport website
Last updated: 27 July 2007