Moisture and Altitude

Psychrometric charts are very useful for determining properties of moist air, but it is important to remember that each chart is prepared for a given atmospheric pressure.  Of course, it is easy to find charts created for specific elevations, but it is sometimes hard to visualize the variations in properties with elevation.  In this post, we’ll look at the effect of elevation on the moisture content of air.

 This figure shows how the humidity ratio (that is the mass of water vapor per unit mass of dry air) varies with elevation for fixed wet bulb and dry bulb temperatures.  Of course, normally as you increase in elevation, the ambient temperature drops, as we talked about here.  However in this case we are specifying the wet bulb and dry bulb temperatures at fixed values, so we are seeing the effect of the decreasing ambient pressure.  That effect of elevation is to significantly increase the amount of moisture per unit mass of air.

This figure shows the relative humidity variation with elevation.  The relative humidity describes the amount of moisture that the air has relative to the maximum amount that it could have before condensation begins.  Again, we have fixed wet bulb and dry bulb temperatures, so the effect is from decreasing ambient pressure.  The relative humidity increases with increasing elevation, but not as drastically as the humidity ratio does over the same range.

It is important to remember that we are not talking about taking a fixed sample of air from a lower to higher elevation, nor are we talking about typical moisture levels as a function of elevation.  Rather, the figures above show the moisture content of the air IF you went to a given elevation AND you measured the given wet bulb and dry bulb temperatures at that elevation.