Tides based on Frequency

The frequency of tides — how many high and low waters occur in a day — is the simplest way to sort them. On this basis, the book recognises three tidal patterns, and the most familiar one is not the only kind.

Semi-diurnal tide — This is the most common tidal pattern in the world. Here, each day brings two high tides and two low tides. What makes it distinctive is that the successive high tides are approximately of the same height, and so are the successive low tides. In other words, the two highs look alike and the two lows look alike, giving a regular, predictable rhythm.

Diurnal tide — As the name suggests, this pattern shows only one high tide and one low tide during each day. Just like the semi-diurnal case, the successive high and low tides are approximately of the same height. So the daily cycle is simpler — a single rise and a single fall.

Mixed tide — These are tides that show variations in height. Unlike the other two patterns, the successive high tides are not equal to each other, and neither are the successive low tides. This irregularity is the defining feature. Geographically, mixed tides generally occur along the west coast of North America and on many islands of the Pacific Ocean.