Counted from the Moon's passage over the local meridian, it is the wait until the next high water (the high water interval) or low water (the low water interval). Averaged over many cycles it is nearly constant for a place, so an almanac time for the Moon's transit plus the interval gives a rough time of high water. The high water interval at full and new moon is called the establishment of the port.
Lunitidal interval is about 4 hours here, so with the Moon on the meridian at 0900, high water's around 1300.
Also heard as
- high water interval
- low water interval
- HWI
- LWI
Related terms
Lunar day
The time for the Moon to return to the same meridian, about 24 hours 50 minutes, which sets the daily tide rhythm.
Subordinate station
A location whose tides are predicted by applying time and height corrections to a nearby reference station.
Phase lag
How far a constituent's local high water trails the corresponding astronomical forcing, given in degrees.
M2
The principal lunar semidiurnal constituent, period 12.42 hours, usually the largest part of the tide.