HUMPHREYS: DIURNAL VARIATIONS 375 



increase most rapidly during the forenoon, and come to a maxi- 

 mum about 10 o'clock. After this the convectional interference 

 decreases, while at the same time the amount of atmosphere in 

 a vertical column of fixed cross section also decreases as a result 

 of expansion and overflow, till at about 4 o'clock in the afternoon 

 the barometric pressure, as we have already seen, has reached a 

 minimum. 



To form some idea of the magnitude of the barometric change 

 due to convectional turbulence, consider the atmosphere between 

 two parallels of latitude near the equator. This limited quantity 

 of the atmosphere may be regarded as a stream flowing around 

 the earth, having its minimum velocity and maximum depth where 

 convection is greatest, and maximum velocity with minimum 

 depth where convection is absent. 



Now the linear velocity of a point on the equator is approxi- 

 mately 28 kilometers per minute, while during the forenoon the 

 rate of increase of the barometric pressure at the same place is 

 roughly 0.2 mm. per hour. Hence a damming up, or check in 

 the flow, of the given stream of atmosphere at the rate of 0.44 

 kilometer per hour would be sufficient of itself to account for 

 the observed rise in the barometer. But if the average velocity 

 of the wind, or flow of the stream in question, is 10 m. per second, 

 which it may well be, the required change in the velocity could 

 be produced by having, during the course of an hour, only one 

 part in eighty of the whole superincumbent atmosphere in touch 

 with the earth, an amount which, from the size of the cumulus 

 clouds, seems altogether reasonable. Besides, the necessary 

 velocity change is of the same order of magnitude as that observed 

 to take place during, and as the result of, vertical convection. 



The afternoon barometric minimum and the forenoon maxi- 

 mum, therefore, are regarded each as an effect of temperature 

 increase; the minimum as due to expansion and consequent over- 

 flow; the maximum as mainly caused by vertical convection and 

 consequent interference with the free circulation of the atmosphere. 



The forced afternoon minimum would occur in an otherwise 

 stagnant atmosphere, and substantially as at present; but not 



