74. Sums, means, and 
curves. An approximate hu- 
midity sum can be obtained 
by adding the absolute hu- 
midities for each of the = 
twenty-four hours, and ex- 
pressing the results in grains 
per cubic foot. It is possible 
to establish a general ratio 
between this sum and the 
transpiration sum of the 
plant, but its value is not 
great at present. Means of 
absolute and of relative hu- 
midity are readily determin- 
able from the psychrograph 
records; the latter are the 
most useful. The mean of 
relative humidity for the 
twenty-four hours of a day 
is the average of the twenty- 
four hour humidities. 
From these means the sea- 
sonal mean is computed in 
the same manner. A close 
approximation, usually with- 
in 1 degree, may be obtained 
in either case by averaging 
the maximum and minimum 
for the period concerned. 
Various kinds of curves are 
of value in -representing 
variation in humidity. Ob- 
viously, these must be 
derived from the psychro- 
graph, or from the psychro- 
meter when the series is 
sufficiently complete. The 
level curve indicates the 
variation in different sta- 
tions at the same time. 
HUMIDITY 47 
= 
Boiling 2!12+;—= - 100 
> a5 
200 
195 Bh 
190 
185 —— 35 
180 
175 80 
170 
165 78. 
160 ; 
185° es 
150 65 
145 
3 140; — 60, a 
-G 135 i) 
< res 
@ 130 55 Pl 
by 125 
120 ~— 50 5 
fo, ts ye WwW 
W 10 a 
rab) aw 
@ 105 40 @ 
&e tao Sil 
@ 95 35 @ 
v0 = 
85 30 
80 
78 = 2s 
bs 20 
65 = 
60 15 
55 
se 10 
As = 
40 i] 
Bh ens 35 = 
Melting Ice 325 ) 
25 P 
20 
1s -——. 10 
10 
5 1$- 
0 
Ss 20 
10 
Pe -—— 28 
20 = 
Fig. 10. Conversion scale for temperatures. 
