Transcription of © NCERTnot to be republished
1 MOVEMENTS OF OCEAN WATERThe ocean water is dynamic. Its physicalcharacteristics like temperature,salinity, density and the externalforces like of the sun, moon and the windsinfluence the movement of ocean water. Thehorizontal and vertical motions are commonin ocean water bodies. The horizontal motionrefers to the ocean currents and waves. Thevertical motion refers to tides. Ocean currentsare the continuous flow of huge amount ofwater in a definite direction while the wavesare the horizontal motion of water. Water movesahead from one place to another through oceancurrents while the water in the waves does notmove, but the wave trains move ahead.
2 Thevertical motion refers to the rise and fall of waterin the oceans and seas. Due to attraction ofthe sun and the moon, the ocean water is raisedup and falls down twice a day. The upwellingof cold water from subsurface and the sinkingof surface water are also forms of verticalmotion of ocean are actually the energy, not the wateras such, which moves across the ocean particles only travel in a small circle as awave passes. Wind provides energy to thewaves. Wind causes waves to travel in the oceanand the energy is released on shorelines.
3 Themotion of the surface water seldom affects thestagnant deep bottom water of the oceans. Asa wave approaches the beach, it slows is due to the friction occurring betweenthe dynamic water and the sea floor. And, whenthe depth of water is less than half thewavelength of the wave, the wave breaks. Thelargest waves are found in the open continue to grow larger as they moveand absorb energy from the of the waves are caused by the winddriving against water. When a breeze of twoknots or less blows over calm water, smallripples form and grow as the wind speedincreases until white caps appear in thebreaking waves.
4 Waves may travel thousandsof km before rolling ashore, breaking anddissolving as wave s size and shape reveal its waves are fairly young ones and areprobably formed by local wind. Slow andsteady waves originate from far away places,possibly from another hemisphere. Themaximum wave height is determined by thestrength of the wind, how long it blows andthe area over which it blows in a single travel because wind pushes thewater body in its course while gravity pulls thecrests of the waves downward. The falling waterpushes the former troughs upward, and : Motion of waves and water moleculesCHAPTER NCERTnot to be republishedMOVEMENTS OF OCEAN WATER121wave moves to a new position (Figure ).
5 The actual motion of the water beneath thewaves is circular. It indicates that things arecarried up and forward as the waveapproaches, and down and back as it of WavesWave crest and trough : The highest andlowest points of a wave are called the crestand trough height : It is the vertical distancefrom the bottom of a trough to the top ofa crest of a amplitude : It is one-half of the period : It is merely the time intervalbetween two successive wave crests ortroughs as they pass a fixed : It is the horizontal distancebetween two successive speed.
6 It is the rate at which thewave moves through the water, and ismeasured in frequency : It is the number of wavespassing a given point during a one-second time periodical rise and fall of the sea level, onceor twice a day, mainly due to the attraction ofthe sun and the moon, is called a of water caused by meteorologicaleffects (winds and atmospheric pressurechanges) are called surges. Surges are notregular like tides. The study of tides is verycomplex, spatially and temporally, as it has greatvariations in frequency, magnitude and moon s gravitational pull to a greatextent and to a lesser extent the sun sgravitational pull, are the major causes for theoccurrence of tides.
7 Another factor is centrifugalforce, which is the force that acts to counterbalance the gravity. Together, the gravitationalpull and the centrifugal force are responsiblefor creating the two major tidal bulges on theearth. On the side of the earth facing the moon,a tidal bulge occurs while on the opposite sidethough the gravitational attraction of the moonis less as it is farther away, the centrifugal forcecauses tidal bulge on the other side ( ).The tide-generating force is the differencebetween these two forces; the gravitationalattraction of the moon and the centrifugal the surface of the earth, nearest the moon,pull or the attractive force of the moon is greaterthan the centrifugal force, and so there is a netforce causing a bulge towards the moon.
8 Onthe opposite side of the earth, the attractiveforce is less, as it is farther away from the moon,the centrifugal force is dominant. Hence, thereis a net force away from the moon. It createsthe second bulge away from the moon. On thesurface of the earth, the horizontal tidegenerating forces are more important than thevertical forces in generating the tidal : Relation between gravitationalforces and tidesThe tidal bulges on wide continentalshelves, have greater height. When tidal bulgeshit the mid-oceanic islands they become shape of bays and estuaries along acoastline can also magnify the intensity of bays greatly change tidalmagnitudes.
9 When the tide is channelledbetween islands or into bays and estuariesthey are called tidal currents. NCERTnot to be republishedFUNDAMENTALS OF PHYSICAL GEOGRAPHY122 Tides of Bay of Fundy, CanadaThe highest tides in the world occur inthe Bay of Fundy in Nova Scotia, tidal bulge is 15 - 16 m. Becausethere are two high tides and two low tidesevery day (roughly a 24 hour period); thena tide must come in within about a sixhour period. As a rough estimate, the tiderises about 240 cm an hour (1,440 cmdivided by 6 hours).
10 If you have walkeddown a beach with a steep cliff alongside(which is common there), make sure youwatch the tides. If you walk for about anhour and then notice that the tide iscoming in, the water will be over yourhead before you get back to where youstarted!Types of TidesTides vary in their frequency, direction andmovement from place to place and also fromtime to time. Tides may be grouped into varioustypes based on their frequency of occurrencein one day or 24 hours or based on their based on FrequencySemi-diurnal tide : The most common tidalpattern, featuring two high tides and two lowtides each day.
