Everyone knows the feeling of stepping outside on a hot summer night. The hot and humid air seems to weigh you down. Compare this to a brisk autumn morning when the cool breeze wakes you up with the help of your warm coffee. These two days have far different air densities, and this phenomenon can be explained using a psychometric chart.
A psychometric chart has axes with the units of moisture content (kg water/kg dry air) vs volume (m^3/kg dry air), or in more plain terms: mass vs volume. Below is an example of what a psychometric chart looks like.
You will notice the green lines display the moisture content as a percentage. This chart displays a dry bulb temperature of 30° C and a relative humidity of 47%. In these conditions, the chart indicates a specific volume of 0.88 m^3/kg dry air, shown in purple. Let’s see what happens when we increase the relative humidity to 80%.
You will notice that the specific volume has increased to 0.89 m^3/kg dry air when the dry bulb temperature remains constant and the relative humidity is increased. This figure shows that at a constant temperature and mass, the volume of a parcel of air will increase as the relative humidity increases. Therefore, the density of that parcel of air will decrease. Thus, at the same dry bulb temperature, a humid day carries air that is less dense compared to a day with lower humidity.
On the hot and humid summer night, you may find yourself saying “the air feels thick out there,” compared to a dry, crisp autumn breeze. As we have just shown, this is an incorrect belief.
Here is another question for you: Is it easier to hit a home run on a humid night or a dry night at the same ball park? Read our next post on the ideal gas law to find out which is the case and why.