Water vapour and water Besides oxygen, water vapour is the most important constituent of the atmosphere, yet, as shown in the table, the amount varies quite a lot. In the colder areas, as for example northern Siberia, the content of water vapour can be as low as 0.01%, but over the tropical oceans it rises to 3% at sea level. Water vapour is important because it can appear in all three physical states: ice (solid), water (liquid) and vapour (gas). We know the usual visual appearance of ice and water, but even these forms can take on strange appear ances at times. Frozen water can appear as snow or hail. Snow can exist in a multitude of different crystalline forms Water almost invariably exists as water drops in the atmos-