We all know that in the world we live in, water exists in 3 forms – liquid, gas and solid. Water vapor refers to the gas form of water and it is produced from evaporation or boiling of liquid water. Liquid water starts to boil when its vapor pressure reaches its surrounding pressure. 
Below is a graph plotting the relationship of water vapor pressure versus temperature. Note that at the normal boiling point of 100°C, the vapor pressure equals the standard atmospheric pressure of 760 Torr or 101.325 kPa (approximately 7.5 Torr per kPa), and that is where evaporative cooling takes place (water turning into gas form takes a part of the heat with it). Anyway, enough of boring details…
So, what is the significance of water vapor in the context of data centers? Well, we first need to understand two terms – relative humidity and dew point.
The picture on the left is an example of a data center on slab floor (this is Sun’s Santa Clara data center). The theory behind cooling such a facility is to bring the cooling closer to the heat source and deliver it horizontally (row-based) or vertically (rack-based), instead of distributing over the sub-floor.
