THE HYDROSPHERE.
More than two thirds of the Earth's surface is covered in water, filling oceans, seas, lakes and rivers. Water is also found underground, in the atmosphere (in the form of a vapour) and in glaciers (in the form of snow and ice). No wonder the Earth is nicknamed the "Blue Planet"! The later of water enveloping the Earth-on, in and above it-is called the hydrosphere. In short, the hydrosphere is the Earth's outer layer of water, uniting water in all its states: liquid, solid and gas.



INDLAND WATERS are all the freshwater bodies found on continents, uniting rivers, lakes and groundwater.
A WATERSHED is an area of land in which all inland waters drain into the same larger body of water.
Many aspects of the sourrounding land and climate can affect how the water flows within a watershed, either slowing down the flow by placing obstacles in the water's path or acceleratings its course.

- topography: the shape, slope and terrain of the area. For example, steeply sloped land drains easily.
- geology: the type, depth and structure of the rock. For example, water flows more easily through the holes and gaps in crushed rock than through compact clay.
- climate: rain or snowfall, winds and temperature. For example, when it rains, the surface-water runoff is slowed by wooded areas on riverbanks. Rainwater flows into streams and rivers less rapidly.
- agriculture, industrial and urban development. For example, a dam can prevent water from flowing freely.

THE OCEANS

The continents divide the ocean waters covering the Earth into five main areas: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Artic Ocean and the Southern Ocean.

Many factors can influence water temperature, including the following:

DEPTH. Example: Sunlight penetrates the upper layers of the oceans and warms the water.
THE SEASONS. Example: In the winter, the ocean loses some of the heat it stored during the summer.
LATITUDE. Example: Surface waters reach mean temperatures between 25 degree Celcius at the equator and between 12 degree Celcius and 17 degree Celcius in temperate zones.

SALINITY is a measure of the amount of salt dissolved in a liquid.

OCEAN CIRCULATION.An ocean current is the movement of seawater in a certain direction. Ocean circulation is the combined effect of all the currents that move across the oceans.


SURFACE CURRENTS are mostly wind-driven. These currents move horizontally, usually in the first 400 m of water below the surface.

SUBSURFACE CUIRRENTS. At depths of more than 800 m, winds no longer affect the ocean circulation. Deep currents prevail. These movements of water are mostly due to variations in density between water layers.

THERMOHALINE CIRCULATION is responsible for major transfers of heat around the world. Without it, the differences in temperature between the equator and the poles would be much more dramatic. In fact, the ocean is as essential as the atmosphere in regulating the Earth's climate.