VisionIAS - Video Classroom Lecture

Geography Class 01

INTRODUCTION (09:03 AM)

A brief overview of the previous class.

OCEANS (09:08 AM)

Temperature and salinity in the ocean:
Surface temperature:
The temperature and salinity vary in the ocean from place to place in a definite way. This causes density variation which ultimately causes the sinking and rising of water leading to movements in the ocean.
Temperatures are moderated away from the coastal areas, and middle of the ocean whereas they are severe near the coastal areas where continentality is high.
Example Hudson Bay, Gulf of Mexico.
Oceans and seas have more severe temperatures in the northern hemisphere than in the southern hemisphere.
Ocean currents bring cold water or warm water they might make the ocean warmer or cooler.
Vertical variation in temperature:
The photic zone gets heated due to sunlight hence in tropical equatorial regions photic zone temperature is higher around 22 to 23 degrees but in temperate regions, the same temperature is 12 to 13 degrees whereas in polar subpolar regions, the temperature might be 2 to 3 degrees.
Just below the photic zone the temperature suddenly starts to decrease to values found at the ocean bottom (2 to 3 degrees).
Hence we find that in tropical regions and temperate regions, there is a sudden considerable drop in temperature from 400m to 1000m after which the value again becomes near constant.
A zone of a sudden change of any value is called as a cline. Hence this sudden change region acts as a thermocline (400m to 1000m).
Polar subpolar region there is no thermocline formation.

SALINITY VARIATION (09:43 AM)

Salt in the ocean means all chemical salts. Salts have two sources. They come from the rocks of the rivers and the bottom of the ocean.
Salinity is measured in parts per thousand.
The average ocean salinity varies between 33 to 37 parts per thousand (ppt). The average is 35 parts per thousand.
The minimum value for saline water is 24.7 parts per thousand. If the salinity is less than this brackish water.
Factors affecting the salinity of the ocean:
Evaporation removes fresh water and increases salinity.
Precipitation adds fresh water and decreases salinity.
Equatorial regions have more precipitation and less salinity.
Temperate regions have less precipitation and more salinity.
The mouth of the river adds fresh water and has less salinity.
In the polar sub-polar regions melting of glaciers and icebergs also reduces salinity.
Ocean currents also modify and change the salinity of the destination region.
Winds also change the salinity by pushing the surface water in a direction.
Enclosed areas have high salinity.
Dead Sea (240 ppt), Lake Van (330 ppt).

VERTICAL SALINITY

Upper part of the waterbody- low salinity and with depth salinity increases.
Salinity high-density high
Salinity low-density low

VARIOUS MOVEMENTS IN THE OCEANS (10:28 AM)

Waves
Tides
Ocean currents
Thermohaline circulations
The wind force acts horizontally pushing the water in the wind direction hence the water accumulates above sea level.
Hence this accumulated water falls back creating a depression below sea level and displacing the water causing the next wave which is now carried by wind. Hence creating high waves and low waves.

TIDES (10:57 AM)

Tide is a daily phenomenon observed on the coast where the water level gradually rises and gradually falls once or twice daily.
Once is called diurnal and if twice is called semi-diurnal.
The rise and fall might be a few feet up to 45- 50 feet or slightly more.
Bay of Fundy, Canada.
Tides are an outcome of the gravitational pull of the sun and the moon and the centrifugal forces due to the rotation of the earth.
It is found that the Moon's gravity is higher than the Sun's gravity because the moon is closer to the Earth.
Both of them (the sun and the moon) try to attract the ocean water in their direction creating bulges towards them and hence a respective depression at 90 degrees.
This rise and fall daily is called high tide and low tide respectively.
Spring tide and neap tide:
On the no moon day and full moon day we have stronger than normal tides because the sun and moon are in the same straight line with respect to earth and hence their gravitational, and centrifugal bulges as well as corresponding depressions, add up.
Whereas on two half-moon days of the cycle, the bulges and depressions of the two cancel each other because the sun and moon are at 90 degrees with respect to the earth.
But because the Moon's gravity is higher therefore we get a weaker-than-normal high tide along the Moon's high tide and lesser than normal low tide along the Moon's low tide.
When we see different coasts of the world they could be classified into one of three types:
1. Semi-diurnal-Eastern coast of India
2. Diurnal tides
3. Mixed semi-diurnal- Western coast of India
These three types are created due to the overlapping of returning tides from different coasts.
Tide creates erosion and deposition on the coast moving the coastal material. They also bring many organisms and take them away in a cyclic way.
They also create intertidal ecosystems.
Tidal navigation and fishing, power.
e.g. Hoogly Port of Kolkata is active during high tides.

CURRENTS (11:50 AM)

Ocean current is the flow of water involving large distances of hundreds to thousands of kilometers, wide- tens to hundreds of kilometers, large volume surface current top 400m, deep ocean current remaining part of 4.5 km.
Surface currents are faster and deep ocean currents are slower.
Unidirectional- slow-moving around the year. Though there are some seasonal currents, they are out of the syllabus.

CAUSES OF SURFACE CURRENTS:

The temperature variation due to Sun's heat
Global winds
Coriolis force
Gravitation

The topic for the next class is surface currents (to continue).