- Books Name
- Social Science Book
- Cognizance Publication
- CBSE Class 7
- Social Science
Water is absolutely essential for survival. About three-fourth of the world’s surface is covered with water. Water is present inrivers, lakes, oceans, etc.
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 formand 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 water bodies
- 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 grounds present in 1000 gram of water. The average salinity of the ocean is 35 parts per thousand.
- Ocean water contains dissolved salts like sodium chloride and hence issaline 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 make it dense.
- Rivers, ponds, springs and glaciers are the major sources of freshwater.
- Less than 1% of the water on Earth is freely available freshwater, and therefore it must be used very carefully and should be conserved.
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.
When the water on the surface of the ocean rises and falls alternately, they are called waves. Waves are found 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.