METEOROLOGY OF AMERICAN ANTARCTIC 



305 



In the high-altitude stations of the Andes the annual variation is 

 reversed: the summer pressure is greater by 2 to 3 millimeters than 

 the winter pressure. 



As one approaches the southern tip of South America the annual 

 variation of atmospheric pressure becomes less regular. At Punta 

 Arenas and in Tierra del Fuego the pressure is 3 to 4 millimeters 

 greater in winter than in summer. At the Falkland Islands the 

 variation is in the same sense, but its amplitude does not exceed 2 

 millimeters. The same annual variation obtains also at South Georgia 

 and the South Orkneys. Elsewhere in the Antarctic the series of 

 observations are too short to allow of any definite deductions. Certain 

 observations, like those made at Petermann Island, indicate greater 

 pressure in summer than in winter, and the march of their annual 

 variation is similar to that of high-altitude stations. 



DIURNAL VARIATION OF ATMOSPHERIC PRESSURE 



Diurnal variation of atmospheric pressure depends on latitude, 

 altitude, and to a less degree on the moisture content of the air. 

 Broadly, the principal maximum of the day takes place at 9 A. M. and 

 the principal minimum between 3 and 5 P. m. A secondary maximum 

 occurs between 10 P. m. and midnight, and a secondary minimum 

 between 3 and 5 a. m. 



This general habitus is illustrated in the subtropical region of 

 South America. There days are very rare on which the barometric 

 curve fails to indicate the diurnal variation, and in the majority of 

 cases the maxima and minima are so marked and regular that the 

 trace of the curve can serve as an indicator of time. 



As one goes southward in latitude the amplitude of the daily 

 variation decreases, as illustrated in the following table: 



In high latitudes the daily variation of barometric pressure is no 

 longer regular; in order to make it evident it is necessary to take the 

 mean of a number of years' observation. 



Also, the principal maximum takes place in the afternoon instead 

 of in the morning. The morning maximum is often hardly noticeable, 

 and the mean daily variation thus becomes a single-period curve with 



