IOWA ACADEMY OF SCIENCE 
187 
The following table gives the amount of precipitation for the corresponding 
months of the year at Iowa City. 
April . . . . 
May 
June 
July 
August . . 
September 
October . . 
3.2 inches 
4.3 inches 
3.0 inches 
,4.5 inches 
,3.3 inches 
,3.5 inches 
,2.4 inches 
It will be seen at once that even in a humid climate like this, that the total 
evaporation from exposed water surfaces is quite comparable with the tota? 
precipitation, possibly as great. 
The relative evaporation during the night to that of the day is perhaps 
larger than might be expected. This is as has already been spoken of, due in 
part to the higher temperature of the water during the night than of the air 
in contact with it. This higher temperature is much more marked for the 
river than the land pan. 
The values found for the relative humidity at seven o’clock in the morning 
and evening are as follows: 
Relative humidity 
1907 7 A. M. 7 P. M. 
April 86% 81% 
May 87 79 
June 88 82 
July 92 88 
August 91 84 
September 92 87 
October 94 83 
1908 
April 89 79 
The temperature of the water in each of the pans and of the river surface 
is of considerable value, giving some clue to the causes of the fluctuations in 
the amount of evaporation. The river temperature is also of interest as it 
shows the march in temperature during the summer months, in the water 
flowing in such a stream. The following table show^s these various tempera- 
tures as well as the average temperature of the air as taken from the mean of 
the readings made at 7 A. M. and 7 P. M. It should be noted that this mean 
is some five degrees lower than the mean as computed from maxima and minima 
readings. 
Land-pan 
River 
-pan 
River Surface 
Mean 
Temp. 
1907 
8 A. M. 
6 P. M. 
8 A. M. 
6 P. M. 
8 A. M. 
6 P. M. 
April 
43.0 
50.0 
46.3 
48.5 
47.3 
48.2 
40.1 
May 
51.6 
61.8 
57.2 
59.7 
57.9 
59.7 
54.4 
June 
66.7 
73.0 
69.0 
71.4 
69.0 
71.8 
65.0 
July 
74.3 
81.0 
75.5 
78.8 
77.3 
78.8 
72.0 
August 
72.0 
78.5 
74.3 
76.5 
75.1 
77.0 
69.0 
September 
61.0 
68.5 
68.2 
69.7 
69.1 
70.0 
61.0 
October _ _ 
50.0 
55.5 
52.7 
55.5 
53.5 
55.7 
47.0 
