FIG. A-7. Air pressure nomogram showing air pressure, temperature, humidity relations. The nomograph involves the equations Pv=Pw–B(Td–Tw)/2700, R=Pv/Pd, W=Pv/1.61(B–Pv), where Pv=actual vapor pressure in inches of mercury (= vapor pressure at the dew point); Pw=vapor pressure at wet-bulb temperature Tw; Pd=vapor pressure at dry-bulb temperature Td; B=barometric pressure; E=elevation in feet; W=specific humidity (= water-vapor weight/dry-air weight); R=relative humidity; ΔT=wet-bulb depression in °F; Tp=dew point. Example: At 750 ft elevation and dry-bulb temperature 95°F, the wet-bulb temperature is 80°F; (1) align 750 ft on scale E with wet-bulb depression ΔT=15°; (2) align where line crosses pivot line with Tw=80° and read Pv=0.87 inches of mercury and dew point=Tp=74.5°; (3) align Pv=0.87 with E=750 ft and read specific humidity=W=0.019; (4) align Pv=0.87 with Td=95° and read relative humidity=52.4%.