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What are Tides and Surges? Explanation about Tides and factors causing them for UPSC CSE.

1. Tides :

The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon is called Tide. Tides occur because of the Gravitational Forces of sun and the moon acting on the earth and also centrifugal force which acts as a counter to it.

The gravitational force of Moon and sun attracts the water from the oceans causing the water level to rise and fall. This is represented in the above animation as bulges that occur on the surface of the earth. The bulge represent the rise in sea level due to attraction force caused by combined gravitational pull from the sun and the moon.

It is a misconception that it is only gravitational force of Sun and the moon that is responsible for causing tides. Centrifugal force caused by the earth to counter the effect of gravitational forces of the sun and the moon which leads to the rise of the sea level also is a factor to be considered when talking about tides generation. Tide-generating’ force = Gravitational attraction force – Centrifugal Force.

2. Spring and Neap Tides :

Spring tides occur when sun and moon are in a straight line. This leads to combined addition of the effect of gravitational pull of both the sun and the moon and hence leading to rise in the sea level. They occur twice a month. Above animation shows how the bulges (ie. the rise in sea level due to gravitational pull ) is larger when the moon and the sun are in the straight line.

Neap Tides occur when the sun and the moon are at right angles to each other. This results in the rise of the sea level but the rise is lower as compared to spring tides, as gravitational pull of both are at right angles and hence do not add or complement to each other completely.

It is important to note here that Gravitational force applied by the moon on the earth is greater than twice that of what is applied by the sun, as the distance of the moon is much near as compared to sun. Near object exert more stronger gravitational force as compared to the far of objects provided mass of both bodies is same.

Gravitational force = (G m1*m2/ r)

m1 being mass of body 1

m2 being mass of body 2

r being the distance between the two bodies. (Now you know why lesser the distance stronger is the gravitational force)

Figure : Formation of Neap Tides.

Figure : Formation of Spring Tides.

2. Important concepts related to Tides:

  • The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada. Two high tides and two low tides every day in Bay of Fundy.

  • The tidal bulges on wide continental shelves, have greater height.

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

  • Flow or flood: The time between the low tide and high tide, when the tide is rising is called Flow.

  • Perigee : When moon orbit is closet to earth it is called Perigee. it results in unusually high and low tides.

  • When the tide is channeled between islands or into bays and estuaries they are called tidal currents.

3. Surges : Movement of water caused by meteorological effects (winds and atmospheric pressure changes) is called a Surge.Surges are not regular like tides.


For more explanations :

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Lecture 2 :

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