Water (NCERT Class 7 Geography Chapter 5 Notes)

Water Cycle/Source: NCERT Class 7 Geography

The Sun causes evaporation of water vapour. The water vapour later cools down and condenses to form clouds. The water drops back on land and surface in the form of rain, snow or sleet. This continuous process is known by the name of the water cycle.

The major sources of fresh water are rivers, ponds, springs, lakes and glaciers. The water of oceans and sea is saline in nature and dissolved with a large number of mineral salts.

Salinity is the amount of salt in grams presents per thousand grams of water. The average salinity of the oceans is 35 parts per thousand. The Dead Sea in Israel has the salinity of 337 parts per thousands whereas the Gaet'ale Pond in Ethiopia with a salinity of 433 per thousand grams is the most saline water body in the world.

Distribution of Water Bodies
Water covers almost seventy-one per cent of the earth surface. However, a very little amount of this water is fit for consumption. Oceans make up 97.3 per cent of total waters. Ice-caps holds 2 per cent of water. Groundwater makes up 0.69 per cent of total water and rest of the water comes from freshwater lakes, inland sea and salt lakes, atmosphere and rivers.

Distribution of Water/Source: NCERT Class 7 Geography

To raise awareness about water, March 22 is celebrated as the Water Day.

Ocean Circulation
The ocean movements can be broadly classified as waves, tides and currents.

Waves

1. When water on the ocean surface rises and falls alternatively, they are called waves.
2. These are formed when winds touch the ocean surface. The stronger the wind, the stronger the waves.
3. Disturbance such as an earthquake can cause huge tidal waves known as     Tsunami.
4. The southernmost tip of India, Indira Point, Andaman and Nicobar Island was submerged due to tsunami after the 2004 Indian Ocean Earthquake.

Tide

1. The rhythmic rise and fall of ocean water are known as the tide.
2. It happens twice a day because of strong gravitational pull exerted by the Sun and the Moon.
3. It is low tide when water falls to its lowest level and recedes from the shore.
4. It is high tide when the water rises to its highest level and covers much of the shore.
5. During New Moon and Full Moon, when the Sun, the Earth and the Moon lined up together in a single line, one observes highest tides, also known as spring tides.
6. When the moon is in the first and last quarter, the ocean waters get pulled in opposite direction as the Sun, the Earth and the Moon are lined up in the diagonally opposite direction, one observes weak tides, also known as neap tides.
7. High tides are useful in oceanic navigation and fish farming.
8. Nowadays, the energy stored in tides is harnessed to make electricity. This sort of energy is known as tidal energy.

Ocean Currents

1. Ocean currents are the streams of water that flowing on the ocean surface in a particular direction.
2. These are of two types: warm and cold.
3. The warm ocean currents originate from the equator and move towards poles. The Gulf Stream is an example of a warm ocean current.
4. The cold ocean currents originate from poles and move towards the equator. The Labrador Ocean Current is an example of a cold ocean current.
5. The areas where both cold and warm currents meet are known as the best fishing grounds in the world. The Sea of Japan and East of North America are such examples. However, this area also experiences fogginess.