The notion of 'tides per day' is actually not quite accurate. Since the moon's day is about 24 hours and 50 minutes, the tides actually occur a little later each day. As this continues over the course of a month there will be times where there is only one high or low tide in a 24 hour period (this is illustrated on the page about tide tables).

Range

Tidal range refers to the difference in height between high and low water.

Diurnal Tides

A single high and a single low tide occur each day. They tend to occur in certain areas when the moon is at its furthest from the equator.

Diurnal tides tend to occur when the moon is furthest from the equator.

Semi-diurnal Tides

Two high and two low waters occur daily and are of relatively similar ranges. This type is more likely to occur when the moon is over the equator.

Semi-diurnal tides tend to occur when the moon is over the equator.

 

 

 

Mixed Tides

These tides also tend to occur as the moon moves furthest north or south of the equator. They are typified by large variances in tidal ranges during the two daily tides.

 

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