Tides and Their Types

At a Glance

• Main cause Gravity of the Moon
• Sun’s effect About 46 per cent of the Moon’s
• Spring tide New moon and full moon; highest range
• Neap tide First and third quarters; lowest range
• Interval High tides about every 12 hours 25 minutes
• Exam link UPSC Prelims, Geography: Ocean Movements, Temperature and Salinity

Where Does It Fit?

This is the fifth topic of the category on ocean movements, temperature and salinity. The previous topics described currents, and this one begins the study of the rhythmic movement of the sea surface. The next topic covers tidal bores and the importance of tides. Previous topic: Effects of Ocean Currents. Next topic: Tidal Bore and Importance of Tides.

What Causes Tides?

In short: the Moon pulls the water on the near side of the Earth, and the spinning of the Earth-Moon pair throws up a bulge on the far side.

Tides are the periodic rise and fall of the sea level, usually twice a day. Their principal cause is the gravitational force of the Moon, which is strongest on the side of the Earth facing the Moon, and weakest on the far side. The Earth and the Moon revolve about a common centre of mass, located inside the Earth, and this motion produces a centrifugal force that is the same everywhere but is balanced against gravity only at the centre. As a result, there are two tidal bulges: one toward the Moon, where gravity exceeds the centrifugal force, and one on the opposite side, where the centrifugal force exceeds the Moon’s pull. The Earth rotates under these bulges, so each place has two high tides and two low tides in a lunar day of about 24 hours 50 minutes. Since the Moon moves about 50 minutes later each day, tides come about 50 minutes later each day, and the interval between two high tides is about 12 hours 25 minutes.

The Sun also produces tides, but its effect is only about 46 per cent of that of the Moon, because it is much farther away, although it is far larger. Tidal force depends on the cube of the distance, and so the Moon, which is nearer, dominates. The tides are therefore a result of the geometry of the Earth, Moon and Sun, as in Eclipses, Lunar Phases and Supermoons.

What Are Spring and Neap Tides?

The term spring has nothing to do with the season. It means a tide that springs up. The tides are also affected by the distance of the Moon. When the Moon is at perigee, its closest point to the Earth, about 357,000 km away, the tidal range is larger by about 20 per cent than normal, giving a perigean tide, and at apogee, the farthest point at about 406,000 km, the tides are weaker. When a new or full moon coincides with perigee, the exceptional perigean spring tides appear. The Earth is also closest to the Sun, at perihelion, in early January, and farthest at aphelion, in early July, and the Sun’s effect varies slightly with it. A very high tide combined with a storm surge is the most dangerous, as in Naming, Warning and Effects of Tropical Cyclones.

What Are the Types of Tides by Frequency?

• Semi-diurnal tide: two high tides and two low tides each day, of nearly equal height; the most common type, found along the Atlantic coasts of Europe and the United States and along the Indian coast.
• Diurnal tide: only one high tide and one low tide each day, of the same height; found in the Gulf of Mexico and parts of Southeast Asia.
• Mixed tide: two high and two low tides each day, with unequal heights; common on the Pacific coast of North America and parts of the Indian coast, such as the east coast.

The tidal range, which is the difference in height between high and low tide, depends on the shape of the coast. Tides are small in the open ocean, about a metre, but are amplified in funnel-shaped bays and estuaries. The Bay of Fundy in Canada has the highest tides in the world, with a range up to about 16 m, and the Gulf of Khambhat and Gulf of Kutch in Gujarat and the Hooghly estuary have among the highest tides in India. Enclosed seas, such as the Mediterranean and the Baltic, have almost no tides. Southampton in England has a double high tide, because of the interplay of the flows in the English Channel and the North Sea. As an analytical point, tides are the only ocean movement caused by forces outside the Earth, which also makes their timing exactly predictable years in advance, in tide tables used by ports and navies.

Why Is This Topic Important?

• Concept Spring and neap asked
• Prediction Port and naval use
• Hazard Storm surge on top of spring tide

What Came Next?

The next topic describes tidal bores and the importance of tides. Next topic: Tidal Bore and Importance of Tides.

Key People and Terms

• Tide Periodic rise and fall of the sea
• Tidal range Difference between high and low tide
• Syzygy Sun, Moon and Earth in line
• Perigee Moon closest to Earth
• Apogee Moon farthest from Earth
• Semi-diurnal Two tides per day

Exam Corner

Mains Angle

Questions ask for causes and types. These points add depth.

• Cause Gravity and centrifugal force
• Pattern Spring, neap, perigean
• Local Range depends on coast shape