A NEW EVAPORATION FORMULA 
23 
in which W is expressed in 0.01 foot, h is the wind velocity at the gage in miles per 
hour as determined by observations at the nearest regular Weather Bureau station, 
and 2x is a quantity dependent upon the location of the gage with reference to the 
lake, the shape of the lake, the depth in every part of the lake, and the direction of 
the wind. Equation (18) is equation (69), page 63 of Publication No. 317. The 
meaning of 2x and the method of computing it for any gage for any wind direction, 
as well as the method of evaluating the constant 0.088 and the exponent 2.4 in 
equation (18) are fully stated in Publication No. 317. 
The values of 2x for the various gages on Lakes Michigan-Huron and Superior 
for the various wind directions are given in Table 9. 
The wind effect for any given wind velocity is proportional to the quantity 2x. 
The maximum wind effect therefore, the wind velocity being the same, at any of the 
four gages shown in Table 9, occurs at the Milwaukee gage for an east or a west 
wind. The minimum wind effect at any of the four gages shown occurs at the 
Marquette gage for a north or south wind. The maximum wind effect at Mackinaw, 
the nearest gage to Marquette, occurs with a northeast or southwest wind (2x = 
— 0.64 or +0.64, respectively). The maximum wind effect at Marquette occurs 
with an east or west wind (2%= +0.40 or —0.40, respectively). In the computa- 
tions of wind effects by equation (18) at Mackinaw for the eight months June to 
Table 9.— Values of Xx for use in formula (IS) at the gages indicated 
Direction of 
wind 
At 
Milwaukee 
At 
Harbor Beach 
At 
Mackinaw 
At 
Marquette 
NE 
E 
SE 
+0.10 
+ .95 
+ .25 
+ .36 
- .16 
- .95 
- .25 
- .36 
-0.86 
- .77 
- .05 
+ .35 
+ .86 
+ .77 
+ .05 
- .35 
-0.64 
- .41 
+ .15 
+ .44 
+ .64 
+ .41 
- .15 
- .44 
+ 0.16 
+ .40 
+ .38 
- .02 
- .16 
- .40 
- .38 
+ .02 
S 
SW 
W 
NW 
N 
September of each of 1910 and 1911, the maximum effect computed was 0.001 
foot, and this occurred on only two days in the eight months stated. The average 
wind velocities at the two stations are not greatly different. It is 8 and 10 miles 
per hour at Sault Ste. Marie and Marquette, respectively, based upon 22 and 21 
years of record, respectively. It is clear, therefore, that the wind effects at Mar- 
quette are less than 0.001 foot on practically all of the days. Accordingly the wind 
effects at that gage were neglected, and the observed elevation of the surface of Lake 
Superior at Marquette was corrected for barometric effects only in the computation 
of h. 
Another picture of the character of the wind effects at Marquette may be had 
by an inspection of the nodal lines and depths for Lake Superior shown on Plate 
3. From an inspection of this plate, together with the values of 2x for Marquette 
shown in Table 9, it may be seen that for all axes, i. e., all wind direction except 
north and south, the nodal line comes near to Marquette; so near that the wind 
effect at the gage is practically a local effect within the bay or bight in which the 
gage is located for all except north and south winds. That the wind effects at 
Marquette are smaller than at any of the other gages on the Great Lakes is also to 
be expected because of the greater depth of Lake Superior, which is, roughly, 100 
fathoms deep on an average — deeper than any of the other lakes. By the theory de- 
