The sun’s heat causes evaporation of water into vapour. When the water vapour cools down, it condenses and forms clouds. From there it may fall on the land or sea in the form of rain, snow or sleet.
The process by which water continually changes its form and circulates between oceans, atmosphere and land is known as the water cycle.
Our earth is like a terrarium. The same water that existed centuries ago still exists today. The water used to irrigate a field in Haryana may have flowed down the Amazon River a hundred years ago.
The major sources of fresh water are the rivers, ponds, springs and glaciers. The water of the oceans is salty or saline as it contains large amount of dissolved salts. Most of the salt is sodium chloride or the common table salt that you eat.
It is an artificial enclosure for keeping small house plants.
Salinity is the amount of salt in grams present in 1000 grams of water.
The average salinity of the oceans is 35 parts per thousand.
Dead sea in Israel has salinity of 340 grams per litre of water.
Swimmers can float in it because the increased salt content makes it dense.
DISTRIBUTION OF WATER BODIES
Three-fourth of the earth surface is covered by water. There is more water than land on this earth still so many countries face water scarcity.
DISTRIBUTION OF WATER BODIES
Saline Water Oceans : 97.3
Fresh Water Ice-caps : 02.0
Ground water : 0.68
Fresh water lakes : 0.009
Inland seas and salt lakes : 0.009
Atmosphere : 0.0019
Rivers : 0.0001
Unlike the calm waters of ponds and lakes, ocean water keeps moving continuously. It is never still. The movements that occur in oceans can be broadly categorised as: waves, tides and currents.
When the water on the surface of the ocean rises and falls alternately, they are called waves.
Waves are formed when winds scrape across the ocean surface. The stronger the wind blows, the bigger the wave becomes.
During a storm, the winds blowing at very high speed form huge waves. These may cause tremendous destruction.
A huge tidal wave : Tsunami
An earthquake, a volcanic eruption or underwater landslides can shift large amounts of ocean water. As a result a huge tidal wave called tsunami, that may be as high as 15m., is formed.
The largest tsunami ever measured was 150m. high. These waves travel at a speed of more than 700 km. per hour. The tsunami of 2004 caused wide spread damage in the coastal areas of India.
The Indira point in the Andaman and Nicobar islands got submerged after the tsunami.
TSUNAMI – THE EARTH’S PANDEMONIUM
Tsunami or the harbour wave struck havoc in the Indian Ocean on the 26 December 2004. The wave was the result of the earthquake that had its epicenter close to the western boundary of Sumatra. The magnitude of the earthquake was 9.0 on the Richter scale.
As the Indian plate went under the Burma plate, there was a sudden movement of the sea floor, causing the earthquake.
The ocean floor was displaced by about 10 – 20m and tilted in a downwardly direction. A huge mass of ocean
water flowed to fill in the gap that was being created by the displacement.
This marked the withdrawal of the water mass from the coastlines of the landmasses in the south and southeast Asia. After thrusting of the Indian plate below the Burma plate, the water mass rushed back towards the coastline.
Tsunami travelled at a speed of about 800km. per hour, comparable to speed of commercial aircraft and completely washed away some of the islands in the Indian ocean.
The Indira point in the Andaman and Nicobar islands that marked the southernmost point of India got completely submerged.
As the wave moved from earthquake epicenter from Sumatra towards the Andaman islands and Sri Lanka the wave length decreased with decreasing depth of water. The travel speed also declined from 700-900km. per hour to less than 70km. per hour.
Indication that tsunami is approaching
The first indication that tsunami is approaching is the rapid withdrawal of water from the coastal region, followed by destructive wave.
When this happened on the coast, instead of people going to high ground, they started assembling at the coast to view the miracle.
As a consequence there was a large casualty of curious onlookers when the gigantic wave (tsunami) struck.
The rhythmic rise and fall of ocean water twice in a day is called a tide.
It is high tide when water covers much of the shore by rising to its highest level.
It is low tide when water falls to its lowest level and recedes from the shore.
The strong gravitational pull exerted by the sun and the moon on the earth’s surface causes the tides.
The water of the earth closer to the moon gets pulled under the influence of the moon’s gravitational force and causes high tide.
Spring Tides & Neap Tides
During the full moon and new moon days, the sun, the moon and the earth are in the same line and the tides are highest. These tides are called spring tides.
But when the moon is in its first and last quarter, the ocean waters get drawn in diagonally opposite directions by the gravitational pull of sun and moon resulting in low tides. These tides are called neap tides.
High tides have benefits High tides help in navigation. They raise the water level close to the shores. This helps the ships to arrive at the harbour more easily.
The high tides also help in fishing. Many more fish come closer to the shore during the high tide. This enables fishermen to get a plentiful catch.
The rise and fall of water due to tides is being used to generate electricity in some places.
Ocean currents are streams of water flowing constantly on the ocean surface in definite directions.
The ocean currents may be warm or cold.
Generally, the warm ocean currents originate near the equator and move towards the poles.
The cold currents carry water from polar or higher latitudes to tropical or lower latitudes.
The Labrador Ocean current is cold current while the Gulf Stream is a warm current.
The ocean current influence the temperature conditions of the area.
Warm currents bring about warm temperature over land surface.
The areas where the warm and cold currents meet provide the best fishing grounds of the world. Seas around Japan and the eastern coast of North America are such examples.
The areas where a warm and cold current meet also experience foggy weather making it difficult for navigation.