58 
JOHN H. EHLERS 
and a light breeze, the maximum is 4.51° C. and the average 347° C, 
as compared with a maximum of 7.95° C. and an average of 6.09° C. 
under full sunlight (Table IV). 
(3) The Effect of Diffuse Light. — Even on dark, cloudy, winter days, 
pine leaves absorb sufficient radiant energy to maintain a temperature 
slightly above that of the air. On a few very dark days the potenti- 
ometer gave zero displacement, indicating that the leaf temperature 
was the same as that of the air, but, as a rule, the leaf was found to 
be from 0.34° C. to a little more than 2° C. higher than the air tempera- 
ture, depending upon the brightness of the diffuse light. In only 
one case was the leaf temperature found to be lower than that of the 
air during the hours of daylight. This occurred on January 29, when 
the air temperature was comparatively high. The average for the 
days given in Table X is 0.95° C. 
Table X 
January 18, 1914 
Cloudy. Light breeze. Relative humidity 77% 
2 
3 
4 
5 
6 
7 
8 
9 
Time of 
Obser- 
vation 
No. of 
Obser- 
vations 
Aver. 
Bridge 
Reading 
in Cm. 
Calculated 
Temp. 
Diff. 
Shade 
Temp. 
Black Bulb 
in Vacuum 
Diff. 
Columns 
5 and 6 
Actual 
Leaf 
Temp. 
Wind 
Velocity 
Miles 
per Hr. 
12.30 
12.48 
9 
3 
9.6 
8.2 
1.16° 
.99 
— 2.2° 
— 2.0 
9.4° 
7.2 
11.6° 
9.2 
— 1.04° 
— 1. 01 
Average 
1.08 
— 2.1 
— 1.02 
January 2j, 191 4 
Cloudy. Breeze moderate. Relative humidity 100% 
I 
2 
3 
4 
5 
6 
7 
8 
9 
10.15 
10.33 
5 
6 
5 
II. 0 
4.9 
5-1 
1.31° 
.58 
.61 
4.0° 
5-1 
6.0 
17.8° 
14.4 
15-6 
13-8° 
9-3 
9.6 
5-31° 
5-68 
6.61 
1.2 
2.4 
2.4 
1 Average 
.83 
5-03 
5.86 
February 19, 1914 
Cloudy. Breeze moderate. Relative humidity 89.5% 
I 
2 
3 
4 
5 
6 
7 
8 
9 
12.38 
1.30 
5 
5 
12.9 
17.8 
1.58° 
2.17 
-6.0° 
-5-5 
12.8° 
17.2 
18.8° 
22.2 
-4.42° 
-3.33 
3-0 
2.4 
Average' 1.88 
-5.75 
-3.87 
