brine drainage cavities. Ice crystallizes In the hexagonal form, and individual 

 subcrystal platelets are usually disc-shaped normal to the principal, or crystallographic, 

 c-axis. Platelet width, and usually crystal width also. Is measured parallel to the 

 direction of the c-axIs. 



Seawater with a salinity of 30 to 35 parts per thousand begins to freeze at 

 about -1.8°C; the Ice sheet then grows at a rate that Is primarily dependent upon 

 the air temperature. As the ice sheet grows, brine cavities form that are elliptical 

 or circular in plan view and elongate vertically. The temperature and salinity of 

 the Ice determines the brine content, and together with density, the air content 

 (Anderson, 1958, p. 148). 



STUDY AREA 



Location 



McMurdo Sound is located at the western extremity of the Ross Ice Shelf and 

 Is part of the Ross Sea that Is bounded on the east by Ross Island, the west by the 

 mountains of Victoria Land, and the south by the Ross Ice Shelf. McMurdo Station 

 and Scott Base are located on Ross Island near the tip of Hut Point Peninsula (Figure 1). 

 The camp occupied by personnel of the U. S. Naval Civil Engineering Laboratory 

 (NCEL) Is on the Ross Ice Shelf 2 miles southeast of Scott Base. The study area was 

 located on the embayment south of Hut Point Peninsula. 



Climate 



The climate of the McMurdo Sound area is characterized by low temperatures, 

 extreme temperature fluctuations, frequent high winds, and drifting snow. Mean 

 annual temperatures for the years 1956 to 1961 vary from -18.5°C (-1.3°F) to 17.1°C 

 (1.8 F), with a mean for the 6 years of -17.8°C (-0.1°F). The coldest temperatures 

 occur In July and August, and vary from -40°C (-40°F) to -50°C (-59°F). Maximum 

 temperatures occur in December and January, and can be as high as 5.5°C (42°F) 

 (Climatology of McMurdo Sound, 1961). 



The freezing index is the number of degree days during a freezing season and 

 is one of the best methods of expressing the duration and Intensity of cold. The 

 degree days for any one day equals the difference between the average daily air 

 temperature and 0°C (32°F) (Linell, 1953, p. 19). The mean air freezing index at 

 McMurdo Station is approximately 13,300 degree days. At Point Barrow, Alaska, 

 the freezing index is 8,500 degree days (Pewe and Paige, 1963, p. 366). Other 

 factors, such as hours of sunshine and solar radiation, are also important in defining 

 the climate of a region and are summarized for McMurdo Sound by Paige and Lee 

 (1965). 



