Chapter 5 — Water

The water cycle is the process by which water continually changes its form and circulates between oceans, atmosphere, and land. The sun's heat causes evaporation; water vapour cools and condenses to form clouds; then it falls as rain, snow, or sleet.

The major sources of fresh water are rivers, ponds, springs, and glaciers. The oceans and seas contain salty (saline) water and are not sources of fresh water.

Oceans contain 97.3% of Earth's total water, but it is saline and not suitable for drinking or irrigation.

Salinity is the amount of salt in grams present in 1000 grams of water. The average salinity of the oceans is 35 parts per thousand. Most of this salt is sodium chloride — the common table salt.

The Dead Sea in Israel has a salinity of 340 grams per litre of water. This makes it so dense that swimmers can float in it easily.

Waves are formed when 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.

A tsunami is a huge wave — as high as 15 m — caused by an earthquake, volcanic eruption, or underwater landslide that shifts large amounts of ocean water. The word 'tsunami' is Japanese and means 'harbour waves'. These waves can travel at more than 700 km per hour.

The tsunami struck on 26 December 2004. The earthquake had its epicentre near the western boundary of Sumatra at magnitude 9.0 on the Richter scale. Tsunami waves travelled at about 800 km per hour, killing more than 10,000 people. Indira Point in the Andaman and Nicobar Islands got completely submerged. The worst-affected Indian areas were Andhra Pradesh, Tamil Nadu, Kerala, Puducherry, and the Andaman and Nicobar Islands.

The rhythmic rise and fall of ocean water twice in a day is called a tide. It is high tide when water rises to its highest level covering much of the shore, and low tide when water falls to its lowest level and recedes from the shore.

Tides are caused by the strong gravitational pull exerted by the sun and the moon on Earth's surface. Water closer to the moon gets pulled by the moon's gravity, causing high tide.

Spring tides are the highest tides and occur during full moon and new moon days when the sun, moon, and earth are in the same line. Neap tides are low tides that occur when the moon is in its first and last quarter, with the sun and moon pulling ocean water in diagonally opposite directions.

High tides help ships arrive at harbours more easily by raising the water level near shores. They help fishermen get a plentiful catch as more fish come closer to shore. The rise and fall of water due to tides is also used to generate electricity in some places.

Ocean currents are streams of water flowing constantly on the ocean surface in definite directions. They may be warm or cold. Warm currents originate near the equator and move towards the poles; cold currents carry water from polar latitudes towards the tropics. The Gulf Stream is a warm current and the Labrador Ocean current is a cold current.

The best fishing grounds are where warm and cold ocean currents meet. The seas around Japan and the eastern coast of North America are such examples. However, these areas also experience foggy weather that makes navigation difficult.

March 22 is celebrated as World Water Day to reinforce the need to conserve water.

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