Types of Tides

Tides are not uniform across the globe. Their frequency, direction, and height change from place to place and from one time to another. Because of this, tides are classified in two main ways: by how often they occur in a day, and by the positions of the sun, moon, and earth relative to one another.

Tides Based on Frequency

The first grouping depends on how many high and low tides occur within a 24-hour period.

• Semi-diurnal tide — This is the most common tidal pattern. It features two high tides and two low tides each day. The successive high tides are roughly equal in height, and the successive low tides are also roughly equal in height.
• Diurnal tide — Here, there is only one high tide and one low tide during each day. As with the semi-diurnal type, the successive high and low tides are approximately of the same height.
• Mixed tide — These are tides that show variations in height. They generally occur along the west coast of North America and on many islands of the Pacific Ocean.

Tides Based on the Sun, Moon, and Earth Positions

The height of the rising water at high tide varies appreciably depending on where the sun and moon stand in relation to the earth. Spring tides and neap tides fall under this category.

Spring tides occur when the sun, the moon, and the earth are in a straight line. In this alignment, the height of the tide is higher than usual. These tides occur twice a month — once during the full moon period and once during the new moon period.

Neap tides occur when the sun and moon are at right angles to each other. In this position, the forces of the sun and moon tend to counteract one another. The moon's attraction, though more than twice as strong as the sun's, is diminished by the counteracting force of the sun's gravitational pull. There is normally a seven-day interval between spring tides and neap tides.

Beyond these two, the book also describes how the distance of the moon and the earth from each other, and from the sun, affects tidal range.

• When the moon's orbit brings it closest to the earth — a position called perigee — unusually high and unusually low tides occur. This happens once in a month, and the tidal range during this time is greater than normal.
• Two weeks later, when the moon is farthest from the earth — a position called apogee — the moon's gravitational force is limited, and the tidal ranges are less than their average heights. …