Understanding Relative Humidity


Figure 1.  The capacity of air to hold water vapor as a function of temperature.  Warmer air can hold much more water vapor.  Data from http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/watvap.html#c1

The graph above displays the amount of water vapor that air can hold (the saturation amount) at different temperatures.  For example, at 20 deg C, 1 cubic meter of air can contain up to 17.3 gm of water vapor before it becomes saturated.

If the relative humidity at 20 deg C is 50 %, then the air is half-way saturated and 1/2(17.3 ) or 8.65 gm/m3.  See the graphic example below.

If the amount of water vapor in the air at 10 deg C is 6 gm/m3, then the relative humidity is  (6/9.4) = 0.638 = 63.8 %.  See below.
Figure 2.  The air at 20 deg C is 50 % saturated, so the relative humidity at 20 deg C is 50 % as represented by the shaded part of the yellow bar.  The air at 10 deg C is 63.8% saturated, or the relative humidity at 10 deg C in this example is 63.8 %.