The Sun's heat causes evaporation of water vapour. When the water vapour cools down, it condenses and forms clouds. From there, it may fallon 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 is known as the water cycle.

Water is life-line of human beings. Its scarcity may create numerous problems but its absence would definitely lead to non-existence of all the living beings on the Earth. It is a precious resource of the nature. We drink water whenever we feel thirsty. We use water in numerous activities such as washing clothes, cleaning house floors, watering garden etc. Industries also need water for their functioning. Thus, water is very essential for human beings and must be conserve.

Tsunami is a Japanese word that means 'Harbour waves' as the harbours get destroyed whenever there is tsunami. An earthquake, a volcanic eruption or underwater landslides can shift large amounts of ocean water. As a result, a huge tidal wave called tsunami, that may be as high as 15 m, is formed.
The largest tsunami ever measured was 150 m high. These waves travel at a speed of more than 700 km per hour. The tsunami of 2004 caused widespread damage in the coastal areas of India. The Indira Point in the Andaman and Nicobar Islands got submerged after the tsunami.

High tides are important for various reasons
(i) They raise the water level close to the shores. This helps the ships to arrive at the harbour more easily.
(ii) The high tides also help in fishing.
(iii) Many more fish come closer to the shore during the high tide. This enables fishermen to get a plentiful catch.
(iv) The rise and fall of water due to tides is being used to generate electricity in some places.

Ocean currents are streams of water flowing constantly on the ocean surface in definite directions which may be warm or cold.

i) 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 a cold current while the Gulf Stream is a warm current.
ii) 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 world.
iii) Seas around Japan and the Eastern coast of North America are such examples. The areas where a warm and cold current meet also experience foggy weather making it difficult for navigation.