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To the meteorologist, moisture or water vapour is one of the most important components of the atmosphere. It not only provides the supply for clouds and rainfall, but it also plays a vital role in energy exchanges within the atmosphere, which affect the Earth's energy balance.

When water is heated to 100C it boils and evaporates into the air. Water at 100C contains enough heat to break all the bonds which link together the molecules in liquid water. However, even at temperatures below 100C some evaporation from water surfaces takes place, with water molecules breaking free and escaping into the air. These free water molecules are collectively known as water vapour or moisture.

There is always water vapour present in the atmosphere. Water vapour enters the atmosphere by evaporation from all surface bodies of water. These include puddles, ponds, streams, rivers, lakes and oceans. Water also enters the atmosphere by transpiration from the leaves of plants and trees.

When air containing water vapour cools in the atmosphere, for example when it rises, the water vapour condenses to form tiny droplets of liquid water (or ice) in clouds. Eventually the water stored within clouds is returned to the Earth's surface in precipitation - rain, hail, sleet or snow - where it is returned to the soil for uptake by vegetation or to surface streams, rivers and lakes and ultimately the sea. This cycle of evaporation, condensation and precipitation is called the water cycle.

Water vapour in the atmosphere behaves like a greenhouse gas, trapping heat that is trying to escape from the Earth to space. The presence of water vapour in the atmosphere significantly strengthens the Earth's natural greenhouse effect.