July, 1922] STEVENS — ENVIRONMENTAL TEMPERATURES 
mean temperature of the air. In some cases the difference amounted to 
as much as 13 or 14 degrees F. (pp. 210, 211). 
That the fluctuations in the temperature of twigs exposed to varying 
conditions of sunHght and shade may be rapid and of considerable extent 
is evident from parts of tables i and 2. The figures there given, however, 
by no means represent extreme conditions. The most extreme fluctuations 
yet observed have been those of small dead twigs lying on the ground 
in full sunlight on days when the sun in an otherwise clear sky is from 
time to time obscured by drifting clouds. Naturally, when the clouds are 
not too large, are drifting at a considerable rate of speed, and are separated 
by fairly wide, clear spaces, the fluctuations in temperature are most rapid 
and have the longest range. Under such conditions the temperatures given 
in table 4 were recorded. Even more rapid fluctuations in temperature 
might be expected on very small twigs, and in measuring the surface tem- 
peratures of leaves Mrs. Shreve^ has noted temperature changes of from 
one to three degrees C. within from 20 to 60 seconds. With a moderately 
strong wind blowing, the change amounted to five degrees in 30 seconds. 
Table 4. ■ Temperatures in Degrees Centigrade of Birch Twigs on Overlook Mountain^ 
Woodstock, N. Y., May ij, IQ21, during a Period of Alternating 
Sunlight and Shade due to Drifting Clouds 
9:15 to 9:30 A.M. 
Air temperature in the 
shade during this time 
varied from 10.5° C. to 
13° c. 
Black Birch 
(Betula lenta L.) 
22 Shade 
30 Shade 
19 Shade 
27 Sun 
17 Shade 
30 Sun 
940 to 10:00 A.M. 
Air temperature in the shade dur- 
ing this time varied from 10° C. to 
12° C. 
Yellow Birch 
(Betula lutea Michx.), 
Very Thin Bark 
36 Sun 
27 Shade 
33 Sun 
29 Partial shade 
35 Sun 
White Birch 
{Betula alba L.), 
Thicker Bark 
24 Sun 
17 Shade 
20 Sun 
18 Partial shade 
23 Sun 
That the conditions under which the data recorded in table 4 were 
taken are extremely favorable for rapid fluctuation in temperature is shown 
by numerous unpublished observations made by Dr. H. L. Shantz on 
solar radiation at Akron, Colorado. Briefly summarized, his observations 
show, as indicated by figure i , that the amount of heat received by a given 
area in intervals of sunlight between cumulus clouds is greater in calories 
than the same area would receive if there were no clouds in the sky. The 
explanation of this condition offered by Dr. Shantz is that the amount 
of heat received in the clear intervals between clouds is augmented by 
reflection from the surfaces of the clouds themselves. 
^ Shreve, Edith B. Apparatus for determining the temperature of leaves, 
book Carnegie Inst. Wash. 17:80-81. 19 18. 
Year- 
