Chapter 5 - Water

• Water is absolutely essential for survival.
• About three-fourth of the world's surface is covered with water.
• World Water Day is celebrated on March 22.
• Our earth is like a terrarium, which is an artificial enclosure for keeping small house plants.
• Water cycle is the process where water continually changes its form and circulates between oceans, atmosphere and land.
  • The sun’s heat causes evaporation of water vapour.
  • When the water vapour cools down, it condenses and forms clouds.
  • Clouds then precipitate into rain, snow, sleet, etc. and come down back to Earth.
  • This continual movement of the same water is called the water cycle.

Distribution of Water and the Major Bodies of Water

• About 97% of all the water on Earth is salty (saline) and about 3% is drinkable (fresh).
• More than 97% of water is in the oceans, 2% is locked in ice-caps and glaciers, and less than 1% is in rivers, lakes, ponds, wells, atmosphere etc.
• 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.
• Ocean water contains dissolved salts like sodium chloride and hence is saline and cannot be used for drinking.
• Dead Sea in Israel has salinity of 45 parts per thousand. Swimmers can float in it because the increased salt content makes it dense.
• Rivers, ponds, springs and glaciers are the major sources of fresh water.
• Less than 1% of the water on the Earth is freely available freshwater and therefore it must be used very carefully and should be conserved.

Circulations in Oceans

• Ocean water moves continuously, unlike the stagnant water of pond and lakes. This continuous movement is known as ocean circulation. Some of the major ocean movements are waves, tides, and currents.
1. Waves
1. Waves are formed when the water on the surface of the ocean rises and falls alternately. It is formed when winds scrape across the ocean surface. The stronger the wind blows, the bigger the waves become.
2. An earthquake, volcanic eruptions or underwater landslides can move a lot of sea water forming enormous tsunami.
3. Tsunami is a Japanese word that means ‘Harbour Waves’.
4. A tsunami is a series of waves that can be more than 100 feet high.
5. Tsunami that struck South and South-East Asian coasts in 26 December 2004, in the Indian Ocean, caused huge damage. The wave was the result of the earthquake having its epicenter close to the western boundary of Sumatra and the magnitude of the earthquake was 9.0 on the Richter scale. It travelled at a speed of about 800 km/hour. India, Indonesia, Sri Lanka and Thailand were some of the countries where there was maximum destruction. The Indira point, the southernmost point of India, in the Andaman and Nicobar islands got completely submerged and the coastal areas of Tamil Nadu, Andhra Pradesh, Puducherry, Kerala and the Andaman and Nicobar Islands were affected the most.
6. Tsunamis are rare in the Indian Ocean as the seismic activity is less as compared to the Pacific Ocean.

 

2. Tides
1. Tides are formed by the rising and falling of the sea due to the attraction of the Moon and the Sun.
2. When the sea reaches its lowest level and recedes from the shore it is known as low tide and when it comes further up the shore after reaching its highest level it is known as high tide.
3. The Sun, the Moon and the Earth are in the same line on full moon and new moon days. On full moon day the Earth is between the Sun and the Moon and on new moon day the Moon is between the Earth and the Sun. In both these cases, the gravitational pull of the sun in addition to the gravitational pull of the moon on Earth cause the oceans to bulge a bit more than usual. This results in high tides that are a little higher and low tides that are a little lower than average. These tides, which occur twice each month, are known as spring tides.
4. When are the Moon and Sun form a right angle (90 degrees), ocean waters gets pulled diagonally. This results in high tides being a little lower and low tides getting a little higher than average. These are known as neap tides and occur during the first and third quarter moon.
5. During high tides the water level close to the shores rises. This helps the ships in navigation and to arrive at the harbour more easily.
6. High tides also help fishermen to catch more fish as more fish come closer to the shore during the high tide.
7. In some places this rise and fall of water due to tides is being used to generate electricity.

 

3. Ocean Currents
1. Ocean currents are streams of water flowing constantly on the ocean surface in specific directions.
2. A current is like a huge river within an ocean, flowing from one place to another.
3. They are caused by wind and the differences in temperature and salinity. They're responsible for a vast amount of movement of the water found in the Earth’s oceans.
4. The ocean currents may be warm or cold.
   1. Warm ocean currents like the Gulf Stream originate near the equator and move towards the poles.
   2. Cold ocean currents like the Labrador Current carry water from Polar Regions towards the equator.
5. The temperature conditions of the area are also influenced by the ocean currents.
6. Cold currents bring cold and warm currents bring warm temperature over the surface of the land.
7. Seas around Japan and the eastern coast of North America are examples of areas where the warm and cold currents meet.