# The Ocean's Tides - Causes and Types

The periodical rise and fall of the water level in oceans and sea, once or twice a day, due to the gravitational pull of the sun and the moon, is called a tide. The study of tides is very complex, spatially and temporally, as it has great variations in frequency, magnitude and height.

## The Ocean's Tide

There are three major forces causing an occurrence of tides they are
1. Moon s gravitational pull
2. Sun s gravitational pull.
3. Centrifugal force which acts opposite to gravitational pull of the earth.
• Tides occur due to an imbalance between the various forces acting on the ocean water at a point in time. In general, the tide-generating force is the difference between these two forces; i.e. the gravitational attraction due to the mass of the moon and the centrifugal force due to rotation of the earth.

### MECHANISM

• When the two forces are not in balance, it gives rise to the tide-generating force. The side of the Earth which is closest to the moon has the strongest gravitational attraction towards the moon while water on the other side of the Earth experiences a weaker gravitational force.
• The moons gravitational force has a greater effect than the sun's gravitational force due to the relative distance of moon and sun respectively. The tide-generating force is proportional to the product of the mass of the two bodies but also inversely proportional to the square of the distance between them.
• The tide-generating force due to the Sun is 0.46 that of the moon.

### OTHER FACTORS CONTROLLING TIDES

• Uneven distribution of water over the globe.
• Irregularities in the configuration of the oceans.
• Differences in the coastal topography of a region. For example, Funnel-shaped bays greatly change tidal magnitudes. When the tide is channelled into bays and estuaries they are called tidal currents.

### TYPES OF TIDES

There is no single method to classify them hence they are grouped arbitrarily based on

• Frequency
• The position of the earth, moon, and sun
• The time period between high tide and low tide
• Magnitude

#### Based on frequency

• Semi-diurnal Tide: This is the most common tidal pattern, featuring two high tides and two low tides each day.
• Diurnal Tides: Only one high tide and one low tide each day. The successive high and low tides are approximate of the same height.
• Mixed Tide: Tides having variations in heights are known as mixed tides. They generally occur along the west coast of North America and also in the Pacific Ocean.

#### Based on sun, moon and earth s positions

• Spring Tides: When the position of the sun, the moon, and the earth are aligned in such a way that it forms a straight line, the height of the tide will be higher than normal. These are called as spring tides. It occurs twice in a month-one on the full moon and the other on the new moon.
• Neap Tides: After seven days of spring tides the sun and the moon form a 90-degree angle between each other. The resultant force of gravitation gives rise to a tide of very low magnitude which is termed as the neap tide. It also occurs twice in a month.

#### Based on magnitude

• Perigee: When the moon s orbit is closest to the earth, it is called as perigee. During this period, unusual heights of high and low tide occur.
• Apogee: When the moon s orbit is farthest from the earth, it is called as apogee. Tidal ranges will be much less than average height during this period.
• Perihelion: It is the position where the earth is closest to the sun (around January 3rd). Unusually high and low tides occur at this time.
• Aphelion: It is the position where the earth is farthest from the sun (around July 4th). Tidal ranges will be much less than the average height during this period.

#### The time period between high tide and low tide

The time between the high tide and low tide, when the water level is falling is called the ebb.

The time between the low tide and high tide, when the tide is rising, is called the flow

### IMPORTANCE OF TIDES

• They help in navigation. During high tide when the water level rises, large ships can enter or leave harbour safely. Diamond Harbour in West Bengal and Kandla in Gujarat are examples of such ports.
• They help the ships to travel up the mouth of the river in case of river ports. High tide increases the volume of water at the river ports to a high volume so that large ships can sail in safely and use a retreating tide to leave the port. Hooghly (Kolkata), London and New York are examples of some of the important river ports of the world.
• They help in removal of silt. Rivers wear away and carry down a lot of soil and sand. Tides wash away these deposits and help to keep the mouth of the river clear.
• Saltwater freezes at a temperature that is low than that of fresh water. In cold regions whereas the rivers are frozen in winter, the warmer seawater rushes into the harbours to keep it free from freezing.
• Tidal energy is largely being harnessed to produce electricity as a renewable source of energy.
• They also bring in huge volumes of fish and these areas serve as fishing zones for fishermen without venturing too deep into the sea.

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