JAC Board Class 7 Social Science Notes Geography Chapter 5 Water
→ The heat of sun causes evaporation of water vapour and when it cools down, it condenses and forms clouds. Then it may fall on the land or sea in the form of rain, snow or sleet.
- Water cycle is the process by which water continually changes its form and circulates between oceans and seas, atmosphere and land.
- Our earth is like a terrarium. The water which existed centuries ago still exists today.
- The fresh water majorly comes from the rivers, ponds, springs and glaciers. The ocean bodies and the seas contains salty or saline water as it contains huge amount of dissolved salts. Most of the salt is of sodium chloride.
→ Distribution of Water bodies:
Three-fourth of the earth surface is covered by water. The following table gives the distribution of water in percentage:
| Saline water | Oceans | 97.3 |
| Fresh water | Ground water | 0.68 |
| Ice caps | 2.0 | |
| Fresh water lakes and Inland seas | 0.009 | |
| Salt lakes | 0.009 | |
| Atmosphere | 0.0019 | |
| Rivers | 0.0001 | |
| Total | 100.00 |
→ Ocean Circulation:
In oceans, the movements that occur can be categorised as waves, tides and currents.
→ Waves:
- Waves occurs when the water on the surface of the ocean rises and falls alternately.
- The winds blows at a very high speed forms huge waves during a storm and causes destruction in an enormous way. An earthquake, a volcanic eruption or underwater landslides may shift large amounts of ocean water. The huge tidal wave is known as tsunami which may be as high as 15 m.
- The largest tsunami ever measured was as high as 150 m. and travels at a speed of more than 700 km. per hour.
→ Tides:
- A tide is the periodic, regular rise and fall of ocean water twice in a day.
- When water covers much of the shore by rising to its highest level then it is high tide. When water falls to its lowest level and recedes from the shore then it is low tide.
- On the earth’s surface, the strong gravitational pull exerted by the sun and the moon causes the tides.
- When the sun, the moon and the earth are in the same line during the full moon and new moon days then the tides are highest. These type of tides are known as spring tides.
- The ocean waters get drawn in diagonally opposite directions by the gravitational pull of sun and earth resulting in low tides when the moon is in its first and last quarter. These type of tides are known as neap tides.
- High tides helps in navigation, fishing and to generate electricity in some areas.
→ Ocean Currents:
- Streams of water which flows constantly on the ocean surface in definite directions is known as ocean currents. The ocean currents may be warm or cold.
- The warm ocean currents emerge near the equator and move towards the poles. The cold ocean currents carry water from polar or higher latitudes to tropical or lower latitudes.
- These currents influence the temperature conditions of the area. The Labrador Ocean current is cold current while the Gulf Stream is a warm current.
- The areas where a warm and cold current is present, they experience the foggy weather which makes navigation difficult but provides the best fishing grounds.
- Types of tides in sea or ocean are flood tide and ebb tide. Flood tide are the rise of sea level and the fall in sea level is called the ebb tide.