BAROMETRIC PRESSURES ON THE GREAT LAKES 23 



been computed directly from table No. 1 by simple division. For example, 



for no lag the values of B m and B m from table No. 1 are '- - and 



0/1 



n 



Hence, in table No. 2, - for no lag is shown as 1.48 (= 



24 



Two values of the lag may be determined from table No. 2, one from the 



R R n D 



ratio - or 2 and the other from the ratio or 3 . The discrepancy 



R R R. R 



^wi D m w ** 



between the two values serves as a test of the degree of accuracy with which 

 the lag is determined by taking the mean of the two values. 



VALUES OF C w AND C n . 



In table No. 1 it may be noted that each of the following equations is 

 either exactly or nearly true : 



= C W (36) 



= C W (37) 



= C n (38) 



= C n (39) 



If table No. 2 shows equations (36) to (39) to be true for the particular 

 case under consideration, then C w and C n may be computed most conven- 

 iently directly from these equations, regardless of the lag. 



If the particular case is for no lag, for example, the table shows that two 

 values of C w may be computed, after said lag is known, from the two equa- 

 tions, written from table No. 1 : 



90 4. 



'B m +B m =- -C W = Q.Q75C W and B wl +B m = C w 



&TX. 



For the cases actually encountered in the investigation, equations (36) to 

 (39) were used as being sufficiently accurate. 



From these equations (36) to (39) two values of C w and two values of C n 

 may be determined from each least-square solution. The discrepancy be- 

 tween the two values serves as a test of the degree of accuracy with which 

 C w or C n is determined by taking the mean of the two values. 



ASSUMED UNIFORM CHANGE IN BAROMETRIC GRADIENTS. 



The derivation of the form of observation equations shown as equation 

 (30) on page 20 is based on an assumption which is stated explicitly below, 

 for convenient reference, as assumption No. 3. 



ASSUMPTION No. 3. 



It is assumed that the barometric gradients along parallels and along 

 meridians vary at a uniform rate in each 12-hour interval between the 

 epochs 8 a.m. and 8 p.m. for which the forecast maps show the facts. 



