A tide curve with two closely spaced maxima, or two minima, separated by a small dip or rise. It comes from strong shallow-water overtides adding to the main semidiurnal tide. Southampton's double high water is the best-known example, giving the port an extended period of high water that suits large ships. Some ports instead have double lows, and predictions there are notoriously hard to read.
With the agger at Southampton you get a second high an hour or two later, so the big ships have time.
Also heard as
- double high water
- double low water
- double tide
Related terms
Overtide
A higher-frequency constituent, such as M4, generated when a tide distorts in shallow water.
Stand
The pause at the top or bottom of the tide, when the level hangs nearly still before turning.
MTL
Mean Tide Level: the height midway between the mean high water and mean low water datums.
Tide window
The span of time when water is deep enough, or current slack enough, for a planned vessel move or job.