0KitemanBest Answer 11 years ago The difference is purely one of temperature. Water vapour is the same temperature as the air in which it is spread. Steam is above the local boiling-point of water (usually taken to be 100°C, but varies with local weather and altitude). It is also a general assumption that "a cloud of steam" will be largely steam / water vapour, whereas "water vapour" is typically only a few percent of the volume of the air. Chemically, steam and water vapour are identical.