A NEW EVAPORATION FORMULA 19 
VALUE OF Ic FOR LAKE MICHIGAN-HURON 
Using equation (7) and the adopted value of - ' the expression for run-off 
E w 
becomes 
Run-off = (Rainfall on land) —0.62 (evaporation from water) (9) 
For Lake Michigan-Huron it was estimated from sources partly external to 
this investigation that the rainfall on the land drainage area is 34 inches per year. 
The annual evaporation from the surface of Lake Michigan-Huron was similarly 
estimated at 28 inches per year. Hence from equation (9) the run-off into the lake 
[34 28 "1 
— — — (0.62) =1.39 feet per year. In 
terms of the rise of the surface of Lake Michigan-Huron, the annual run-off is 
1.39 ( j^oTij^ 2.84 feet per year, or (2.84-^365 = ) 0.008 feet per day, which is the 
first approximation to Ic for Lake Michigan-Huron used in this investigation. 
VALUE OF I c FOR LAKE SUPERIOR 
For Lake Superior, the rainfall on the land drainage area was estimated 
at 27 inches per year. The evaporation from the surface of Lake Superior was 
estimated at 23 inches per year. Hence from equation (9) the run-off into the 
lake from the adjacent drainage area, expressed in terms of the land area, is 
[27 23 1 
Y7}~Tq (0.62) =1.06 feet per year. In terms of the rise of the surface of Lake 
Superior, the annual run-off is 
1.06 L ' 6Q )= 1-45 feet per year, 
or 
(1.45 -=-365 = ) 0.004 foot per day, which is the first approximation to Ic for Lake 
Superior used in this investigation. 
RISE IN THE MEAN SURFACE OF THE WHOLE LAKE (A) 
Up to this point, the methods of computing all of the known quantities which 
enter into equation (1), except the quantity I u have been stated in general terms. 
It is now proposed to state briefly the method of evaluating I x . The exposition 
involved in this will necessarily overlap Publication No. 317 to some extent, for 
in that publication the theory involved in the evaluation of wind and barometric 
effects was set forth, as well as some results of the application of corrections for 
those effects. It is not the intention to here restate the theory nor any of the 
discussions of the results in Publication No. 317, but rather to carry over certain 
parts of it only for convenience in the present discussion, and only those parts dealing 
with Lake Michigan-Huron. The material pertaining to Lake Superior to be set 
forth here is herewith published for the first time. 
BAROMETRIC POINTS FOR LAKES MICHIGAN-HURON AND SUPERIOR 
The location of the points from which the barometric pressures were read from 
the forecast maps is given in Table 8, and shown on Plate 2. 
HOURLY BAROMETRIC EFFECTS 
The effect, E\, of unequal barometric pressures on the surfaces of the Great 
Lakes in causing the recorded elevation of the lake surface at the gage for any hour 
to be different from the mean elevation of the whole lake surface for that hour, is 
