GEOGRAPHICAL DISTRIBUTION OF TEMPERATURE— SEA-LEVEL. 



35 



WEST LONG. 160 



FlO. 9. Isotherms foi- July. 



Isotherms for January. — The temperature in McMurdo Sound i« 24° E. and at Cape Adare 

 32°F., it is therefore probable that the temperature over the open water of the Ross Sea 

 is approximately 30°F. The floating ice and ice-covered land will probably reduce the 

 temperature to 25°F. near the south and east of the sea. The 25°F. isotherm has therefore 

 been shown to encircle the sea near to the Barrier and to the permanent ice in the east, 

 with the 30°F. isotherm surrounding a warmer central area. The temperature of the whole of 

 the Barrier i.s within a degree or so of 1.5°F.. which fixes the position of the L5°F. 

 isotherm. The 20°F. isotherm has been drawn between those for 15°F. and 25°F., and 

 we see that the closeness of the isotherms near to the edge of the Barrier indicates the 

 large temperature gradient which must exist between the cold Barrier surface and the 

 relatively warm open sea. 



Isotherms for July. — The temperatures at Cape Evans and at Cape Adare are practically 

 the same during June, July, and August and they are more than 20° above those of the 

 Barrier. We shall show later (see page 90) that during the winter the temperature of the sea ice 

 decides the temperature of the air above it and one knows that the sea ice is much thinner 



