366 Tables 98 and 99 (continued) 



REDUCTION OF PSYCHRO METRIC DATA 



Table 98, Centigrade Temperatures. — Equation (1) can be rewritten 



e = e'- [0.000660(1 + 0.00115O ]p(t-t') (3) 



or e = e' — Ae 



where Ae = [0.000660(1 + 0.00115/') ]p(t — t'), temperatures are in °C, and p, c, and c' 

 are in the same pressure units. Table 98 gives values of Ae in millibars for a pressure p 

 of 1000 mb. for various values of the wet-bulb temperature t' and of the depression of the 

 wet-bulb (* — *')• Since [0.000660(1 -f 0.001 15/' )]p(t — t') is linear in both p and 

 (t — t') values of Ae for another barometric pressure p may be found by multiplying the 

 tabular value by /'/1000. In practice it is sufficient to use the nearest round 100 mb. value 

 so that this operation can be performed by subtraction (e.g., if p = 900 mb., subtract 0.1 

 Ae from Ae, etc.). The linearity in (t — t') enables rapid computations for fractions of a 

 degree depression of the wet-bulb (e.g., if (t — t') =2.4°, Ae is one-tenth the value for 

 24°). For ordinary purposes the t' column for the temperature nearest the wet-bulb 

 temperature may be used ; for extreme precision Ae should be interpolated linearly for t'. 

 Instructions for use of Table 98. — To obtain e, the vapor pressure of the air : 



1. Determine e' the saturation vapor pressure at the wet-bulb temperature 



(a) if the wet-bulb temperature is °C. or above use Table 94 (e' for water) ; 



(b) if the wet-bulb temperature is below °C. use Table 96 (e' for ice). 



2. Determine Ae at a pressure of 1000 mb. (A^iooo) from Table 98. 



3. Reduce Aeiooo to the appropriate pressure p by: 



(a) multiplying A^iooo by /»/1000, or 



(b) substracting 0.1 Aemm from A^ooo if pressure is 900 mb., 0.2 Aciooo if 800 mb., 



etc. 



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



Example.— Given r = 40.1 °C, f = 36.9 °C, (t — f) =3.2 °C, /> = 923 mb. 



1. From Table 94, <?' = 62.421 mb. 



2. From Table 98, with f = 40 °C, and p = 1000 mb. Ac 1M « for (t—f) =32 °C. is 



22.092 mb., therefore for (* — *') =3.2 °C, An™ = 2.209 mb. 



3. For 900 mb. Ae = Ac im — 0.1 Aciooo = 2.209 - 0.221 = 1.988 mb., 



4. e = e'-Ae = 62.421 - 1.988 = 60.433 mb. 



Table 99, Fahrenheit temperatures. — For Fahrenheit temperature (3) can be rewritten 



* = e'-[o.000367(l +^p)]/>(^ -*')' (4) 



or 



e = e — &e 

 where Ae= [0.000367 ( 1 + * ~ 32 )1 p(t — t'), temperatures are in °F., and p, c, and 



e' are in the same pressure units. Table 99 gives values of Ac in inches of mercury for a 

 pressure p of 30 in. Hg. (with auxiliary columns for p = 1 in. Hg.) for various values of 

 the wet-bulb temperature t' and of the depression of the wet-bulb (t — t'). Since Ae is 

 linear in both p and (t — t'), values of Ae for another barometric pressure /> (in. Hg.) 

 may be found by multiplying the value for 1 in. Hg. by p, or by a lineai combination of 

 the 30 and 1 in. Hg. columns. (E.g., if p = 29 in. Hg., subtract the value of Ae for 1 in. 

 Hg. from the value of Ac for 30 in. Hg.). The linearity in (t — t') enables rapid compu- 

 tations for fractions of a degree depression of the wet-bulb (e.g., if (t — t') =2.4°, Ac is 

 one-tenth the value for 24°). For ordinary purposes the t' column for the temperature 

 nearest the wet-bulb temperature may be used, or for extreme precision Ac should be 

 interpolated linearly. 



Instructions for the use of Table 99. — To obtain e, the vapor pressure of the air: 



1. Determine e' the saturation vapor pressure at the wet-bulb temperature 



(a) if the wet-bulb temperature is 32 °F. or above use Table 95, (c' for water) : 



(b) if the wet-bulb temperature is below 32 °F. use Table 97 (e' for ice). 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



