Tables 98 and 99 (continued) 357 



REDUCTION OF PSYCHROMETRIC DATA 



2. Determine Ac for a pressure of 30 in. Hg. (Aft ) and Ae for a pressure 1 in. Hg. 



(Aft). 



3. Reduce to Ae for the appropriate pressure p by : 



(a) multiplying Aft by p (in. Hg.), or 



(b) linearly combining Afto and Aft ; e.g., if p = 28 in. Hg. Ae = Afto — 2Aft. 



4. Subtract Ac from e', the result is the desired vapor pressure. 

 Example.— Given / = 58.5 °F„ f = 54.8 °F., t — t' = 3.7 °F. p = 28.73 in. Hg. 



1. From Table 95, e' = 0.43248 in. Hg. 



2. From Table 99, with t' = 60 °F. and (t—t')= 3.7 °F. Ae for 30 in. Hg. (Aft,) = 



0.41463 in. Hg., Ae for 1 in. Hg. (Aft) =0.01382 in. Hg. and for (/ — t') = 

 3.7 °F. Aft = 0.04146 in. Hg. Aft = 0.00138 in. Hg. 



3. For p = 29 in. Hg. Ae = Afto — Aft = 0.04146 — 0.00138 = 0.04008 in. Hg. 



4. e = e' — Ae = 0.43248 — 0.04008 = 0.39240 in. Hg. 



Dew point. — Resolutions 180 and 184 of the Twelfth Conference of Directors (Wash- 

 ington, 1947) recommend the use of dew point (not frost point) in all synoptic surface 

 and upper air reports including those in circumstances where the vapor pressure is lower 

 than the saturated vapor pressure at °C. (32 °F.). To obtain the dew point correspond- 

 ing to the vapor pressure computed above, enter the body of the table "Saturation Vapor 

 Pressure Over Water" (Table 94 for vapor pressure in millibars, Table 95 for vapor 

 pressure in inches of mercury) and read off the corresponding dew-point temperature. 

 (E.g., if e = 60.433 mb., the dew-point temperature = 36.3 C C, if e = 0.39240 in. Hg., the 

 dew-point temperature = 52.1 °F.) 



Frost point. — Frost-point temperatures may be determined in the same manner as the 

 dew-point temperature; use the tables "Saturation Vapor Pressure Over Ice" (Table 96 

 for vapor pressure in millibars, Table 97 for vapor pressure in inches of mercury). 



Relative humidity. — Resolution 166 of the Twelfth Conference of Directors (Wash- 

 ington, 1947) (see Table 93) redefines the relative humidity as 100 r/r w percent, where r 

 is the mixing ratio of the air at the given pressure and temperature and r w is the satura- 

 tion mixing ratio over water at the same temperature and pressure. This value may be 

 closely approximated by the ratio 100 e/e w where e is the vapor pressure as obtained 

 above and e w is the saturation vapor pressure over water at the dry-bulb temperature. The 

 approximate relative humidity so determined will differ from the true relative humidity 

 at most by 2\ percent under the extreme conditions of about 50 percent relative humidity 

 and a very high temperature of about 50 °C. At percent relative humidity there is no 

 difference and at 100 percent the difference is negligible. At a temperature of 10 °C. and 

 50 percent relative humidity the difference is about 0.5 percent. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



