REPORT ON ATMOSPHERIC CIRCULATION. 



21 



The geographical distribution of this oscillation is given in the accompanying Fig. 2, 

 which shows for July its amount by lines of 10, 20, 40, 60, 80, and 100 thousandths of 



160 MO 120 



80 6 J 40 20 20 40 M 



DO 120 1*0 16 18 



Fig. 2.— Ohart showing the mean monthly amount of the diurnal oscillation of the barometer over the globe for July. 



an inch, or - 010 inch, 0'020 inch, etc. The abnormally small amount over the centre 

 of the Atlantic and the Mediterranean begins in March, attains the maximum in June, 

 and ends in October. It is thus confined to the warmer months of the year, and is not 

 cumulative, like most other meteorological phenomena, but follows the sun, having its 

 maximum in June. The smallness of the oscillation over the North Atlantic, which is 

 probably less than occurs in any other ocean in the same latitude, is to a large extent 

 caused by the small dip in the diurnal curve of the afternoon minimum, thus indicating 

 an atmosphere where the heating by the sun is comparatively small. Over the open sea 

 of the higher latitudes, the afternoon dip, or afternoon minimum, disappears, thus reducing 

 the barometric curve to one maximum and one minimum during the twenty-four hours. 



The much greater amplitude of the oscillations on land, as compared with the open 

 sea, is entirely due to the heating of the surface of the earth, this higher temperature, 

 which has its origin in the superheated surface, being in addition to the direct heating 

 of the air by the heat rays of the sun as they pass through it. Tension is thereby still 

 further increased, and, consequently, the morning maximum and the afternoon 

 minimum are both more extreme than over the open sea. The oscillation reaches its 

 maximum just in those tropical climates where insolation is strongest, and the effect 

 is doubtless still further heightened by the greater number of dust particles present 

 in the atmosphere of these climates. 



In low latitudes, where the velocity of the surface due to the earth's rotation is 

 near the maximum, the four phases of the barometric oscillations are most sharply 

 defined and of greatest amplitude. But in the higher latitudes, where the velocity of 

 the surface is much reduced, the amount of the oscillation is also small, and the one 

 phase passes into the other by easy gradations. 



It has been shown in the Table, p. 13, that in situations more or less insular 



