Water

There are three states of matter - solid, liquid and gas.  To understand weather it is important to know these as they apply to water.

When water is in a gas state, it is called water vapour.

When water is liquid, it's just called water.

When it is solid, it is called ice.

This sounds simple until you starting reading about 'water in the atmosphere'. What could that possibly mean?

'Water in the atmosphere' is a way of saying that there is water vapour in the atmosphere.  We know water is a gas so this means that the water vapour has mixed with the oxygen, nitrogen and other gases in air.  And because water vapour, as a gas, is colourless, we can't see it.  The amount of water vapour in the atmosphere can change depending on the temperature of the atmopshere.

When air cools, water vapour - the gas - condenses into water - the liquid.  We can see this because when it happens, clouds form.  Clouds are very large masses of tiny water droplets.

You've probably already figured from this that whether or not it rains depends on the temperature.

The Water Cycle.

Water is evaporated from water body such as a river, lake, sea or reservoir.  Evaporation converts the water into water vapour (H₂0).  The water vapour rises into the atmosphere.

As the water vapour rises it begins to cool.  As it cools, it condenses.  Condensation turns the water vapour back into water droplets.  Clouds are blown towards land where further cooling occurs.  Eventually water droplets combine and become heavy enough to fall as Precipitation (Rain, Hail, Sleet, Snow). 

When rain falls on land, some of the rain flows back to the sea as Run Off either across land or through rivers.  Infiltration carries water through land where Through Flow carries it to rivers or the sea.  Also, Stem Flow from trees and plants carries water to land and eventually to the sea where it is evaporated again.  This cycle is known as the Water Cycle.

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