Oceanography- 4 | Geography for UPSC 2024 (Pre & Mains) PDF Download

 Page 1


 
 
OCEANOGRAPHY - 4 
TIDES- DEFINITION 
The rise and fall of sea level once or twice a day mainly due to the 
attraction of the sun and moon is called tide.  
 
PLEASE NOTE: 
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 a tide.  
 
Movement of water caused by meteorological effects (winds and 
atmospheric pressure changes) are called surges.  
  
FACTORS RESPONSIBLE FOR TIDES 
The moon’s gravitational pull to a great extent and to a lesser extent the 
sun’s gravitational pull, are the major causes for the occurrence of tides.  
Another factor is centrifugal force, which is the force that acts to counter 
the balance the gravity.  
Together, the gravitational pull and the centrifugal force are responsible 
for creating the two major tidal bulges on the earth.  
On the side of the earth facing the moon, a tidal bulge occurs while on the 
opposite side though the gravitational attraction of the moon is less as it is 
farther away, the centrifugal force causes tidal bulge on the other side  
The ‘tide-generating’ force is the difference between these two forces; i.e. the 
gravitational attraction of the moon and the centrifugal force.  
Page 2


 
 
OCEANOGRAPHY - 4 
TIDES- DEFINITION 
The rise and fall of sea level once or twice a day mainly due to the 
attraction of the sun and moon is called tide.  
 
PLEASE NOTE: 
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 a tide.  
 
Movement of water caused by meteorological effects (winds and 
atmospheric pressure changes) are called surges.  
  
FACTORS RESPONSIBLE FOR TIDES 
The moon’s gravitational pull to a great extent and to a lesser extent the 
sun’s gravitational pull, are the major causes for the occurrence of tides.  
Another factor is centrifugal force, which is the force that acts to counter 
the balance the gravity.  
Together, the gravitational pull and the centrifugal force are responsible 
for creating the two major tidal bulges on the earth.  
On the side of the earth facing the moon, a tidal bulge occurs while on the 
opposite side though the gravitational attraction of the moon is less as it is 
farther away, the centrifugal force causes tidal bulge on the other side  
The ‘tide-generating’ force is the difference between these two forces; i.e. the 
gravitational attraction of the moon and the centrifugal force.  
 
 
On the surface of the earth, nearest the moon, pull or the attractive force of the 
moon is greater than the centrifugal force, and so there is a net force causing a 
bulge towards the moon.  
On the opposite side of the earth, the attractive force is less, as it is farther 
away from the moon, the centrifugal force is dominant.  
Hence, there is a net force away from the moon. 
 It creates the second bulge away from the moon. 
  
TYPES OF TIDES 
TIDES BASED ON FREQUENCY 
Semidiurnal tide: The most common tidal pattern, featuring 2 high 
Tides and 2 low tides each day. The successive high or Low Tides are 
approximately of the same height.  
Diurnal tide: There is only one high tide and one low tide during each 
day. The successive high or Low Tides are approximately of the same 
height.  
Mixed tide: Tides having variation in height are known as mixed tide. 
these tides generally occur along the west coast of north America and on  
many islands of pacific ocean. 
TIDES BASED ON SUN, MOON AND EARTH POSITIONS 
The height of the rising water (high tide) varies appreciably 
depending upon the position of sun and moon with respect to the 
earth.  
 
Page 3


 
 
OCEANOGRAPHY - 4 
TIDES- DEFINITION 
The rise and fall of sea level once or twice a day mainly due to the 
attraction of the sun and moon is called tide.  
 
PLEASE NOTE: 
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 a tide.  
 
Movement of water caused by meteorological effects (winds and 
atmospheric pressure changes) are called surges.  
  
FACTORS RESPONSIBLE FOR TIDES 
The moon’s gravitational pull to a great extent and to a lesser extent the 
sun’s gravitational pull, are the major causes for the occurrence of tides.  
Another factor is centrifugal force, which is the force that acts to counter 
the balance the gravity.  
Together, the gravitational pull and the centrifugal force are responsible 
for creating the two major tidal bulges on the earth.  
On the side of the earth facing the moon, a tidal bulge occurs while on the 
opposite side though the gravitational attraction of the moon is less as it is 
farther away, the centrifugal force causes tidal bulge on the other side  
The ‘tide-generating’ force is the difference between these two forces; i.e. the 
gravitational attraction of the moon and the centrifugal force.  
 
 
On the surface of the earth, nearest the moon, pull or the attractive force of the 
moon is greater than the centrifugal force, and so there is a net force causing a 
bulge towards the moon.  
On the opposite side of the earth, the attractive force is less, as it is farther 
away from the moon, the centrifugal force is dominant.  
Hence, there is a net force away from the moon. 
 It creates the second bulge away from the moon. 
  
TYPES OF TIDES 
TIDES BASED ON FREQUENCY 
Semidiurnal tide: The most common tidal pattern, featuring 2 high 
Tides and 2 low tides each day. The successive high or Low Tides are 
approximately of the same height.  
Diurnal tide: There is only one high tide and one low tide during each 
day. The successive high or Low Tides are approximately of the same 
height.  
Mixed tide: Tides having variation in height are known as mixed tide. 
these tides generally occur along the west coast of north America and on  
many islands of pacific ocean. 
TIDES BASED ON SUN, MOON AND EARTH POSITIONS 
The height of the rising water (high tide) varies appreciably 
depending upon the position of sun and moon with respect to the 
earth.  
 
 
 
Spring tides and neap tides come under this category. 
Spring tides:  
The position of both the sun and the moon in relation to the earth has 
direct bearing on tide height . When the sun the moon and the earth 
are in a straight line the height of the tide will be higher.  These are 
called spring tides and they occur twice a month one on a full moon 
period and another during New Moon period. 
Neap tides:  
At this time the sun and the moon are at right angles to each other 
and the forces of the sun and the moon tend to counteract one 
another.  The moon's attraction, though  more than twice as stronger 
than the sun, is diminished by the counteracting force of the sun's 
gravitational pull.  
Perigee and Apogee: 
Once in a month when the Moon's orbit is closest to the earth 
(perigee) unusually high and low Tides occur.  During this time the 
tidal range is greater than normal  
 
Two weeks later when the moon is farthest from the Earth (apogee), 
the moon's gravitational force is limited and and the tidal ranges 
are less than their average height.  
 
Perihelion and Aphelion 
 
When the earth is closest to the sun, perihelion, around 3rd 
January each year, tidal ranges are also much greater, with 
unusually high and unusually low tides.  
 
Page 4


 
 
OCEANOGRAPHY - 4 
TIDES- DEFINITION 
The rise and fall of sea level once or twice a day mainly due to the 
attraction of the sun and moon is called tide.  
 
PLEASE NOTE: 
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 a tide.  
 
Movement of water caused by meteorological effects (winds and 
atmospheric pressure changes) are called surges.  
  
FACTORS RESPONSIBLE FOR TIDES 
The moon’s gravitational pull to a great extent and to a lesser extent the 
sun’s gravitational pull, are the major causes for the occurrence of tides.  
Another factor is centrifugal force, which is the force that acts to counter 
the balance the gravity.  
Together, the gravitational pull and the centrifugal force are responsible 
for creating the two major tidal bulges on the earth.  
On the side of the earth facing the moon, a tidal bulge occurs while on the 
opposite side though the gravitational attraction of the moon is less as it is 
farther away, the centrifugal force causes tidal bulge on the other side  
The ‘tide-generating’ force is the difference between these two forces; i.e. the 
gravitational attraction of the moon and the centrifugal force.  
 
 
On the surface of the earth, nearest the moon, pull or the attractive force of the 
moon is greater than the centrifugal force, and so there is a net force causing a 
bulge towards the moon.  
On the opposite side of the earth, the attractive force is less, as it is farther 
away from the moon, the centrifugal force is dominant.  
Hence, there is a net force away from the moon. 
 It creates the second bulge away from the moon. 
  
TYPES OF TIDES 
TIDES BASED ON FREQUENCY 
Semidiurnal tide: The most common tidal pattern, featuring 2 high 
Tides and 2 low tides each day. The successive high or Low Tides are 
approximately of the same height.  
Diurnal tide: There is only one high tide and one low tide during each 
day. The successive high or Low Tides are approximately of the same 
height.  
Mixed tide: Tides having variation in height are known as mixed tide. 
these tides generally occur along the west coast of north America and on  
many islands of pacific ocean. 
TIDES BASED ON SUN, MOON AND EARTH POSITIONS 
The height of the rising water (high tide) varies appreciably 
depending upon the position of sun and moon with respect to the 
earth.  
 
 
 
Spring tides and neap tides come under this category. 
Spring tides:  
The position of both the sun and the moon in relation to the earth has 
direct bearing on tide height . When the sun the moon and the earth 
are in a straight line the height of the tide will be higher.  These are 
called spring tides and they occur twice a month one on a full moon 
period and another during New Moon period. 
Neap tides:  
At this time the sun and the moon are at right angles to each other 
and the forces of the sun and the moon tend to counteract one 
another.  The moon's attraction, though  more than twice as stronger 
than the sun, is diminished by the counteracting force of the sun's 
gravitational pull.  
Perigee and Apogee: 
Once in a month when the Moon's orbit is closest to the earth 
(perigee) unusually high and low Tides occur.  During this time the 
tidal range is greater than normal  
 
Two weeks later when the moon is farthest from the Earth (apogee), 
the moon's gravitational force is limited and and the tidal ranges 
are less than their average height.  
 
Perihelion and Aphelion 
 
When the earth is closest to the sun, perihelion, around 3rd 
January each year, tidal ranges are also much greater, with 
unusually high and unusually low tides.  
 
 
 
When the earth is farthest from the Sun, aphelion, around 4 July 
each year, tidal ranges are much less than average.  
 
TERMS: 
 
Ebb: 
The time between the high tide and low tide, when the water level is 
falling, is called the ebb.  
 
Flow: 
 
The time between the low tide and high tide, when the tide is rising, 
is called the flow or flood. 
Tidal bore 
A tidal bore , is a tidal phenomenon in which the leading edge of the 
incoming tide forms a wave (or waves) of water that travels up a river or 
narrow bay against the direction of the river or bay's current. 
 
 
IMPORTANCE OF TIDES 
NAVIGATION 
Since tides are caused by the earth-moon-sun positions which are 
known accurately, the tides can be predicted well in advance. This 
helps the navigators and fishermen plan their activities. Tidal flows 
are of great importance in navigation. Tidal heights are very important, 
Page 5


 
 
OCEANOGRAPHY - 4 
TIDES- DEFINITION 
The rise and fall of sea level once or twice a day mainly due to the 
attraction of the sun and moon is called tide.  
 
PLEASE NOTE: 
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 a tide.  
 
Movement of water caused by meteorological effects (winds and 
atmospheric pressure changes) are called surges.  
  
FACTORS RESPONSIBLE FOR TIDES 
The moon’s gravitational pull to a great extent and to a lesser extent the 
sun’s gravitational pull, are the major causes for the occurrence of tides.  
Another factor is centrifugal force, which is the force that acts to counter 
the balance the gravity.  
Together, the gravitational pull and the centrifugal force are responsible 
for creating the two major tidal bulges on the earth.  
On the side of the earth facing the moon, a tidal bulge occurs while on the 
opposite side though the gravitational attraction of the moon is less as it is 
farther away, the centrifugal force causes tidal bulge on the other side  
The ‘tide-generating’ force is the difference between these two forces; i.e. the 
gravitational attraction of the moon and the centrifugal force.  
 
 
On the surface of the earth, nearest the moon, pull or the attractive force of the 
moon is greater than the centrifugal force, and so there is a net force causing a 
bulge towards the moon.  
On the opposite side of the earth, the attractive force is less, as it is farther 
away from the moon, the centrifugal force is dominant.  
Hence, there is a net force away from the moon. 
 It creates the second bulge away from the moon. 
  
TYPES OF TIDES 
TIDES BASED ON FREQUENCY 
Semidiurnal tide: The most common tidal pattern, featuring 2 high 
Tides and 2 low tides each day. The successive high or Low Tides are 
approximately of the same height.  
Diurnal tide: There is only one high tide and one low tide during each 
day. The successive high or Low Tides are approximately of the same 
height.  
Mixed tide: Tides having variation in height are known as mixed tide. 
these tides generally occur along the west coast of north America and on  
many islands of pacific ocean. 
TIDES BASED ON SUN, MOON AND EARTH POSITIONS 
The height of the rising water (high tide) varies appreciably 
depending upon the position of sun and moon with respect to the 
earth.  
 
 
 
Spring tides and neap tides come under this category. 
Spring tides:  
The position of both the sun and the moon in relation to the earth has 
direct bearing on tide height . When the sun the moon and the earth 
are in a straight line the height of the tide will be higher.  These are 
called spring tides and they occur twice a month one on a full moon 
period and another during New Moon period. 
Neap tides:  
At this time the sun and the moon are at right angles to each other 
and the forces of the sun and the moon tend to counteract one 
another.  The moon's attraction, though  more than twice as stronger 
than the sun, is diminished by the counteracting force of the sun's 
gravitational pull.  
Perigee and Apogee: 
Once in a month when the Moon's orbit is closest to the earth 
(perigee) unusually high and low Tides occur.  During this time the 
tidal range is greater than normal  
 
Two weeks later when the moon is farthest from the Earth (apogee), 
the moon's gravitational force is limited and and the tidal ranges 
are less than their average height.  
 
Perihelion and Aphelion 
 
When the earth is closest to the sun, perihelion, around 3rd 
January each year, tidal ranges are also much greater, with 
unusually high and unusually low tides.  
 
 
 
When the earth is farthest from the Sun, aphelion, around 4 July 
each year, tidal ranges are much less than average.  
 
TERMS: 
 
Ebb: 
The time between the high tide and low tide, when the water level is 
falling, is called the ebb.  
 
Flow: 
 
The time between the low tide and high tide, when the tide is rising, 
is called the flow or flood. 
Tidal bore 
A tidal bore , is a tidal phenomenon in which the leading edge of the 
incoming tide forms a wave (or waves) of water that travels up a river or 
narrow bay against the direction of the river or bay's current. 
 
 
IMPORTANCE OF TIDES 
NAVIGATION 
Since tides are caused by the earth-moon-sun positions which are 
known accurately, the tides can be predicted well in advance. This 
helps the navigators and fishermen plan their activities. Tidal flows 
are of great importance in navigation. Tidal heights are very important, 
 
 
especially harbours near rivers and within estuaries having shallow ‘bars’ 
at the entrance, which prevent ships and boats from entering into the 
harbor 
FISHING 
The high Tides also help in fishing. Many more fish come closer to the 
Shore during high tide.  This enables fishermen to get a plentiful 
catch. 
DESILTING 
Tides are also helpful in desilting sediment and in removing polluted 
water from river estuaries 
POWER 
Tides are used to generate electrical power (in Canada France Russia 
and china) .  
 
FACTORS CONTROLLING THE NATURE AND MAGNITUDE OF 
TIDES 
The movement of the moon in relation to the earth. 
Changes in position of the sun and moon in relation to the earth. 
Uneven distribution of water over the globe. 
Irregularities in the configuration of the oceans. 
The tidal bulges on wide continental shelves, have greater height. When 
tidal bulges hit the mid-oceanic islands they become low. 
The shape of bays and estuaries along a coastline can also magnify the 
intensity of tides.