water vapor / steam

Water vapor, also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation of liquid water or from the sublimation of ice. Under normal atmospheric conditions, water vapor is continuously evaporating and condensing.


圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Water vapor condensing over a cup of hot tea

Steam most often refers to the white mist that condenses above boiling water as the hot vapor ("steam" in the first sense) mixes with the cooler air. This mist is made of tiny droplets of liquid water, not gaseous water, so it is no longer technically steam.



Difference between Steam and Water Vapour
Steam is merely water vapour at a temperature equal to or greater than the boiling point. We cannot see water vapour; for the same reason, we cannot see steam.
When water is heated in a vessel with a nozzle, a cloud forms as the hot water vapour issuing from the nozzle mixes with the colder surrounding air. Such a cloud is popularly, although incorrectly, called “steam”. We cannot see steam, so what we can see is not rightly called steam. Call the cloud what you like, but just remember that it is composed of water droplets not water vapour. Like mixing cloud in meteorology, it is the mixing of moist air masses with different characteristics that yields such a cloud. If you heat the nozzle with a gas torch the cloud will soon disappear. Yet, water vapour is still issuing from the nozzle.

I will consider that the temperature of them are different, as Steam must have over 100C, it is the vapour form of water. But water vapour may not be at high temerature, it could be in vapour form when the relative humidity is low in the atmosphere then the water will be evaporated to be a small droplets of vapour form.