METEOROLOGY OF AMERICAN ANTARCTIC 303 



South Orkneys and the South Shetlands already are polar islands 

 almost completely covered by ice and snow. As for Graham Land 

 and the lands that continue it towards the south, Adelaide Island, 

 Fallieres Land, Alexander I Land, and Charcot Land, a conception 

 of their general aspect can be gained if one envisages what would 

 happen to any mountainous country bordering the sea with steep- 

 sloped mountains of 2000 to 3000 meters elevation were it covered by a 

 sheet of ice several tens of meters thick. Toward the south glaciation 

 is still more intense, and all relief is drowned under an immense ice cap 

 from which project only the steep summits of individual peaks. The 

 underlying rock appears only in places where the walls are too steep 

 for snow to hold. 



Atmospheric Pressure 



In the following general summary of the meteorology of the Ameri- 

 can quadrant of the Antarctic we will consider only the three principal 

 elements of climate, viz. atmospheric pressure, temperature, and wind 

 movement. 



MEAN ATMOSPHERIC PRESSURE 



The mean values of atmospheric pressure decrease rapidly with 

 increasing latitude. In Argentina in latitude 40° S. the mean atmos- 

 pheric pressure reduced to sea level is 760 millimeters; in latitude 45° 

 it is only 757 milhmeters; in 50°, 753 millimeters; in 55°, 748 milli- 

 meters. The mean value of atmospheric pressure at Punta Arenas 

 in latitude 53° 10' S., as deduced from a long series of observations, is 

 751.3 millimeters. Over Tierra del Fuego as over Patagonia the mean 

 isobars follow the direction of the parallels. 



But as one proceeds eastward into the Atlantic the isobars curve 

 slightly toward the north. Thus the mean atmospheric pressure in 

 South Georgia, situated in latitude 54° 20', is 746.7 millimeters, or 

 4.6 millimeters lower than the mean at Punta Arenas. If only the 

 years are considered during which simultaneous records were made at 

 both stations, the difference between Punta Arenas and South Georgia 

 becomes 5 millimeters. The mean barometric pressure is practically 

 the same at the Falkland Islands in 51° 30' and at Punta Arenas in 



53° 10'. 



As we proceed south the pressure decreases. Again taking the 

 value at Punta Arenas as a starting point, we find that the atmospheric 

 pressure in the South Orkneys in latitude 60° 44' is 7.8 millimeters 

 lower than at Punta Arenas. We thus have seen that between latitudes 

 50° and 55° pressure diminishes by 5 millimeters, i. e. by one millimeter 

 per degree of latitude. This rate of decrease continues to latitude 

 60°. 



