• The sun’s heat causes evaporation of water 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 to known as the water cycle.
• Our earth is like a terrarium.
• The major source of fresh water are the rivers, ponds, springs and glaciers.
• The ocean bodies and the seas contain salty water.
• Distribution of Water on Earth:
(i) About three-fourth of the earth’s surface is covered by water.
(ii) The following table gives the distribution of water in percentage:

(iii) Water is absolutely essential for survival.
• Movements:
(i) Unlike the calm waters of ponds and lakes, ocean water keeps moving continuously.
(ii) The movements which occur in oceans are of three types: waves, tides and currents.
• Waves:
(i) When the water on the surface of the ocean rises and falls alternately, they are called waves.
(ii) An earthquake, a volcanic eruption or underwater landslides can shift large amounts of ocean water. As a result, huge tidal wave may be formed which is called tsunami.
(iii) Tsunami in South and South-East Asian coast had caused havoc in December 2004.
• Tides:
(i) The rhythmic rise and fall of ocean water twice in a day is called a tide.
(ii) Tides are are two types: spring tides and neap tides.
• Ocean Currents:
(i) Ocean currents are streams of water flowing constantly on the ocean surface in different directions.
(ii) Ocean currents are of two types, warm and cold.
(iii) The Labrador ocean current is a cold current, while the Gulf Stream is a warm current.