10 
A NEW METHOD OF ESTIMATING STREAM-FLOW 
assumptions made in the investigation of evaporation, see Publication No. 317, 
pages 12, 16, 23 and 43.) 
Assumption No. 5 — It is assumed that the mean temperature of the surface 
water of the lake for a day, during the months of May to October inclusive, is the 
same as that of the free-air above, as measured at the U. S. and Canadian Weather 
Bureau stations nearest the lake. 
Assumption No. 5 was necessarily made in the absence of, and the practical 
impossibility of obtaining, data on the mean daily temperature of the surface waters 
of the Great Lakes. It was made for convenience both in the study of evaporation 
and in the subsequent study of stream-flow. 
The temperature of the surface water of a lake changes more slowly than that 
of the air above, hence it will in general be colder during the day and warmer during 
the night than the overlying air in the summer months. The mean day and night 
temperature of the surface water will, in general, tend to be the same as that of the 
overlying air. 
In a lake of the size and depth of any of the Great Lakes, the volumetric 
temperature of the lake will tend to lag behind the air temperature; that is, in the 
early spring and summer months the air temperature will generally be warmer than 
the temperature of the whole mass of water in the lake, and in the late summer and 
fall the reverse will, in general, be true. The statements made in Assumption No. 
5, and in the preceding paragraph, refer to the surface film of water from which 
evaporation takes place, and not to the whole mass of water in the lake. 
METEOROLOGICAL STATIONS NEAR LAKES MICHIGAN-HURON AND SUPERIOR 
The regular U. S. Weather Bureau and Canadian Meteorological Service 
stations at which observations of wind velocity, vapor-pressure and air temperature 
were recorded are shown in Table 1 for Lake Michigan-Huron and in Table 2 for 
Lake Superior. These stations are also shown plotted on Plates I and 4 respec- 
tively. 
Table 1 — Regular U. S. Weather Bureau and Canadian Meteorological Service Stations near Lake 
Michigan-Huron 
Station 
In the United States: 
Chicago 
Milwaukee 
Grand Haven . . . 
Green Bay 
Escanaba 
Sault Ste. Marie. 
Alpena 
Port Huron 
In Canada: 
Parry Sound . 
Saugeen 
Means 
Latitude 
41° 53' 
43 02 
43 05 
44 31 
45 48 
46 30 
45 05 
43 00 
45° 19' 
44 30 
Longitude 
87° 37' 
87 54 
86 13 
88 00 
87 05 
84 21 
83 30 
82 26 
80° 00' 
81 21 
Elevations in feet 
Barometer 
above 
sea 
823 
681 
632 
617 
612 
614 
609 
638 
635 
656 
652 
Thermometer 
above 
ground 
140 
122 
64 
49 
40 
40 
13 
70 
4.6 
4.5 
64 
Rain 
gage 
above 
ground 
133 
116 
49 
43 
33 
35 
4 
63 
1 
1 
48 
Anemometer 
above 
ground 
310 
140 
89 
86 
82 
61 
92 
120 
40* 
44* 
106 
* Approximate elevation of ground above mean lake level, 42 feet and 62 feet, respectively. 
