BAROMETRIC PRESSURES ON THE GREAT LAKES 



15 



water surface along a meridian is called the " N-S slope, " positive when it is 

 upward to the northward. Equation (9) is true, because the water surface 

 under assumption No. 1 is an inclined plane. 



]&By substitution in equation (7) of the values of MI M c and of Hi-H c 

 from equations (8) and (9) there is obtained 



Ei= -(W-E slope) (LJ- (N-S slope) (L n ) 



= +(W-E gradient) (LJ (1.13)+ (N-S gradient) (L n ) (1.13) (10) 



Or, dropping out the middle part of (10), which is no longer necessary after 

 the conceptions are grasped 



-E gradient)(L w )(1.13)+(N-S gradient) (L B )( 1.13) (11) 



EXPRESSION OF BAROMETRIC GRADIENTS. 



From a study of the forecast maps and of the general conditions of the 

 problem, it was decided that the best feasible way to secure satisfactory 

 values for the W-E gradient and the N-S gradient for each of the Great 

 Lakes at the many times for which they were needed was as follows : 



(1) Eleven points were selected for which 

 readings were to be taken from the forecast 

 maps to cover all of the Great Lakes region. 

 The six of these points used in connection with 

 Lake Erie and Lake Michigan-Huron, known 

 as points 3, 4, 5, 6, 7, and 8, are shown on the 

 sketch on the lower half of plate 2. Their exact 

 locations are shown in the adjoining table: 



(2) The values of the barometric pressure 

 were read directly from the forecast maps for 



8 a.m. and 8 p.m. (75th meridian time) of each day and tabulated in con- 

 venient form. 



(3) Let " (6-8) " stand for the barometric pressure at point 6 minus the 

 barometric pressure at point 8 for a given time. Let " (distance 6 to 8) " 

 stand for the distance from point 6 to point 8. Note, that points 6 and 8 

 are on the same parallel, that they are both in the latitude of Lake Erie, 

 point 6 to the westward and point 8 to the eastward. The (W-E gradient) 

 for Lake Erie was then taken as 



(6-8) 



(distance 6 to 8) 



(4) Similarly, the (N-S gradient) for Lake Erie was taken as 



(5-7) 

 (distance 5 to 7) 



(5) For Lake Michigan-Huron, the (W-E gradient) was taken as 



(4-5) 

 (distance 4 to 5) 



(12) 



(13) 



(14) 



