374 HUMPHREYS! DIURNAL VARIATIONS 



velocity, U, in the same direction, would be given by the equation, 



?nv +MV 

 m + M 



and there obviously would be no check in the total flow, no dam- 

 ming up and consequent increase of pressure. But this simple 

 mixing of the two masses is by no means the whole story. The 

 rise of the mass m is simultaneously accompanied by the descent 

 of an equal amount from the larger mass M. Thus from a single 

 interchange, due to vertical convection, the total momentum be- 

 comes 



2mv + (M - m)V 



Hence the total flow is reduced, through ground friction by the 

 amount 



m(V — v) 



But as this is for a single interchange, it is obvious that the more 

 active vertical convection becomes, the greater will be its inter- 

 ference with the flow of the atmosphere, the more the winds will 

 be dammed up and the higher the resulting barometric pressure. 

 In general, as convection increases, reaches a maximum and then 

 decreases, so too will the resulting interference go through the 

 same changes. 



Now the general movement of the atmosphere is from east to 

 west within the tropics and from west to east at higher latitudes. 

 Hence in either case such damming up of the atmosphere as verti- 

 cal convection may produce will be essentially along meridians, 

 just as any given phase of vertical convection itself, which has 

 to be substantially at right angles to the march of the sun, is 

 also essentially along a meridian. In other words, convection 

 and its attendant phenomena are functions of the time of day. 

 But, in general, convection increases most rapidly during the fore- 

 noon, say 8 to 9 o'clock, is most active at 10 to 11 o'clock, and 

 reaches its greatest elevation about 4 o'clock in the afternoon. 

 Hence the damming up of the atmosphere, due to vertical con- 

 vection, and the resulting increase of barometric pressure must 



