Absolute Humidity is the vapor content of air, given in grams or lb of water vapor per lb of dry air (i.e., lb of moisture/lb of d.a). It is also known as moisture content or humidity ratio. Air at a given temperature can support only a certain amount of moisture and no more. This is referred to as the saturation humidity.

Humidity ratio is represented on the chart by lines that run horizontally and the values are on the right hand side (Y-axis) of the chart increasing from bottom to top.