Potential Vapor Pressure and its Applications

Abstract

It has been pointed out that the mixing ratio may be computed from the formula: where W is the mixing ratio expressed in grams per kilogram of dry air, e, the vapor pressure, and p, the pressure of air. It is wellknown that the mixing ratio is one of the most conservative quantities that can be used for purpose of airmass identification, and the mixing ratio is constant in an adiabatic process. Thus it is quite eyident that the relation between pressure and vapor pressure in an adiabatic displacement of an unsaturated particle is as follows: where e is the vapor pressure of the particle at the pressure p, and e' is the vapor pressure it assumes at the pressure p' Thus we have an important relation as follows: If we now specify that the particle, originally at the vapor pressure e and the pressure p', be compressed to 1000_??_mb pressure, we have where E is the vapor pressure, which the particle assumes at this pressure 1000mb, and we shall call E as the potential vapor-pressure. The potential vapor-pressure is als_??_ constant in an adiabatic process. By definition it is clear that where E is the original potential vapor-pressure of the particle at the pressure P and the vapor pressure e, and E' is the potential vapor-pressure it assumes at the pressure p' and the vapor pressure e'. By means of Eq. (1) it can be easily shown that E=E' Thus proposition is proved.
After vapor pressure h_??_s been determined, the potential vapor-pressure, E, is calculated by means of the formula (2). A slide rule is most convinient for this computation. (Table giving the factor 1000/p may also be constructed to facilitate this computation.) It can be shown that potential temperature and potential vapor pressure can be used for the purposes of air mass identification. These two quantities can also be used in isentropic analysis and synoptic weather science.
For example, in any adiabatic chart, the lines of constant potential vapor pressure are identical with the lines of equal values of the humidity mixing ratio, and they may be called as the dew-point lines. This fact facilitates the computation of the dew-point lines. Further, when an element of air is set in motion in dry adiabatic process, the dr_??_ adiabatic through the dry-bulb temperature, the saturated adiabatic through the wet-bulb temperature and the dew-point line through the dew-point in any adiabatic chart, all meet in a point at the condensation level.