NCERT class VII : Our Environment – Water – Chapter 5
Points to Remember
The process by which water continually changes its form and circulates between oceans, atmosphere and land is known as the water cycle.
The major sources of fresh water are the rivers, ponds, springs and glaciers. The ocean bodies and the seas contain salty water(Presence of sodium chloride).
Three-fourth of the earth surface is covered by water.
The movements that occur in oceans can be broadly categorised as:
- Waves: Waves are formed when gentle winds scrape across the ocean surface. The stronger the wind blows, the bigger the wave becomes. When the water on the surface of the ocean rises and falls alternately, they are called waves.
- Tides:The rhythmic rise and fall of ocean water twice in a day is called a tide. The strong gravitational pull exerted by the sun and the moon on the earth’s surface causes the tides.
- High Tide:It is high tide when water covers much of the shore by rising to its highest level.The water of the earth closer to the moon gets pulled under the influence of the moon’s gravitational force and causes high tide.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. They 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.
- Low Tide: It is low tide when water falls to its lowest level and recedes from the shore.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 earth resulting in low tides. These tides are called neap tides.
The rise and fall of water due to tides is being used to generate electricity in some places.
- Currents- 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