184 



SCIENCE 



[N. S. Vol. XLIX. No. 1260 



{d) The amplitude is everywhere greatest 

 oil equinoxes and everywhere least on solstices. 



(e) The amplitude is greater at perihelion 

 than at aphelion. 



(/) The amplitude is greater by day than 

 by night. 



(g) The amplitude is greatest on clear days 

 and least on cloudy days. 



(70 The day amplitude is greater over land 

 than over water. 



(•i) The night amplitude is greater over 

 oceans than over continents. 



(j) Over the tropical Pacific Ocean the fore- 

 noon maximum is about 1 mm. above, and the 

 afternoon minimum 1 mm. below the general 

 average pressure. 



Taken together these facts compel the con- 

 clusion that the daily cyclic pressure changes 

 are somehow the results of temperature 

 changes. Many efforts have been made to 

 find just how the two are connected — how, 

 for instance, a diurnal change in temperature 

 can produce a semidiurnal change in pressure, 

 but until recently without much success. To 

 be sure, the diurnal temperature curve is not 

 a simple sine curve of twenty-four hours 

 period, and, like any other curve, can be 

 closely duplicated by a series of superimposed 

 sine curves of proper amplitudes and periods. 

 But this is far from satisfying to the average 

 physicist, and particularly so if he has not 

 yet seen by what process the actuail un- 

 analyzed temperature changes can produce 

 corresponding pressure variations in the open 

 atmosphere. Of course it may be argued that 

 there must always be a flow of air from the 

 warmer to the colder regions, and, therefore, 

 a pressure wave of diurnal period perpetually 

 s^\-eeping around the earth. Such a wave does 

 indeed appear in the analyzed data. And as 

 the temperature curve yields sine waves of 

 twenty-four hours, twelve hoiu's, eight hours 

 and other periods, so, therefore, may the pres- 

 sure curve also. But while this may be a 

 portion of the story, it certainly is not the 

 whole of it. There are other and important 

 connections between temperature changes and 

 pressure variations that must be considered; 

 and these indeed seem to be the chief factors 



in producing the semidiurnal pressure wave. 



One of these factors is vertical convection, 

 first suggested, but never adequately devel- 

 oped, by Cleveland Abbe. Indeed, it appears 

 to be the principal cause of the forenoon max- 

 imum as the following consideration shows: 



Let the mass m of air be near the ground 

 and have the horizontal velocity v, and let the 

 larger mass M be at a higher elevation and 

 have the greater velocity V in the same di- 

 rection. If now these two masses should 

 mingle in such manner as to be free from all 

 disturbance, except their own mutual inter- 

 ference, the resulting final velocity, U, in the 

 same direction, would be given' by the equa- 

 tion 



mv + MV 



U= 



m -\- M 



and there obviously would be no check in the 

 total flow — no damming up and consequent in- 

 crease of pressure. But this simple mixing 

 of the two masses is by no means all that 

 happens in the case of vertical convection. 

 The rise of the mass m is simultaneously ac- 

 companied by an equivalent descent of air 

 from a higher level, which in turn loses 

 velocity, directly or indirectly, by surface 

 friction. If the falling mass is also m, and if 

 its velocity is reduced by friction to v, then 

 from a single interchange, due to vertical con- 

 vection, the total momentum, becomes 



2 mv + {M — m)V 



and the total flow is reduced by the amount 



m{'V — v). 



But as this is for a single interchange, it is 

 obvious that the more active vertical con- 

 vection 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 barometric pressure. As convec- 

 tion increases, reaches a maximum, and then 

 decreases, so, too, will the resulting interfer- 

 ence go through the same changes. 



Now the general movement of the atmos- 

 phere is from east to west within the tropics 

 and from west to east at higher latitudes. 

 Therefore in either case such damming up 



