BAROMETRIC PRESSURES ON THE GREAT LAKES 19 



Similarly, the line marked "B 2 No Lag" in plate 3 represents the variation 

 in the elevation of the water surface which will occur if (6-8) increases by 

 an amount b w at a uniform rate from 8 p.m. of the preceding day to 8 a.m. 

 of the current day. By the same process of reasoning which was used above 

 in connection with b m , it may be shown that the increase in mean elevation 

 for the current day over the mean elevation for the preceding day will be 



- b m C w j b m C w = - -7- b m C w = b Wt B wt (22) 



24 24 24 



m which 



B m = C (23) 



24 



So, too, the line marked " B 3 No Lag" on plate 3 represents the variation 

 in elevation if (6-8) increases by an amount b m at a uniform rate from 8 

 a.m. of the current day to 8 p.m. of the current day. In this case the 

 increase in the mean elevation for the current day over the mean elevation 

 for the preceding day will be 



w = b m B wt (24) 



24 

 in which 



(25) 



The line marked "B No Lag" on plate 3 represents the variation in eleva- 

 tion if (6-8) increases an amount b m at a uniform rate from 8 p.m. of the 

 day before the preceding day to 8 a.m. of the preceding day. In this case 

 the increase in the mean elevation for the current day over the mean eleva- 

 tion for the preceding day will be 



in which 



C=&A, (26) 



24 



24 



Note that B m , B Wl , B m , and B m (see equations 27, 21, 23, and 25) are 

 all constants to be derived from the least-square computation. Each one 

 involves the proportionality factor P w and other values which are different 

 at different stations but do not vary with the lapse of time. Consult equa- 

 tions (19) and (17). 



Normally, (6-8) is found to be different at each of the successive epochs 

 8 a.m. and 8 p.m. at which the barometric pressures are determined from 

 the forecast maps. Hence, on the assumptions which have been stated, the 

 total decrease, due to this cause, in the mean elevation of the current day 

 over the mean elevation for the preceding day will be 



m B wt (28) 



