There are several different methods for defining the amount of water vapor in the atmosphere. The definition used depends on the application.

Boiling point decreases with altitude - boiling occurs when saturation vapor pressure of escaping bubbles is greater than the total atmospheric pressure.

Humidity and clouds

To form clouds we need to change the phase of the atmospheric water vapor to either a liquid or a solid. To do this we have to increase the relative humidity to near 100%. We can accomplish this by either cooling the air or by evaporating water vapor into the air. Most clouds form by cooling the air through lifting.

From the hydrological cycle we know atmospheric water vapor content is the surface. Water must evaporate (or sublime) from the surface. The rate of evaporation is a function of temperature of the water, the temperature of the air, the relative humidity of the air, the wind speed and the surface area of the water. Think of the bathroom hand-dryers: