208 HOBBS— ROLE OF GLACIAL ANTICYCLONE [April 24. 



we are indebted especially to Professor David, the meteorologist of 

 the Shackleton expedition, for a careful study of the Antarctic 

 type of blizzard as observed by him at the winter station of the ex- 

 pedition. I shall here cite my earlier summary of the sequence of 

 events with some personal interpretations.*^ 



" The sequence of events during a blizzard begins with gentle northerly 

 winds which continue for a day or two during which temperatures are low. 

 David has suggested that during this time air is flowing south to take the 

 place of air whose volume has been reduced as a result of the heat ab- 

 stracted from it on the ice surface. Then there follow two or three days of 

 absolute calm, during which the temperature continues to fall. Still further 

 cooled upon the ice surface, the air, a week or more after the calm begins, 

 starts to move outward in all directions and so develops (on the edge of the 

 barrier) a southeasterly blizzard. Simultaneously with this movement the 

 steam cap over the volcano of Erebus, which normally indicates an upper 

 current from the northwest, swings round to the north and takes on an 

 accelerated movement, as though it were being drawn from that direction 

 to supply air to the void resulting from the violent surface current toward 

 that direction. Corresponding to the increased velocity, the normal foehn 

 efifect near the pole must be much increased as it is also on the descent of the 

 surface current from the plateau. As soon as the warming of the polar air 

 from this cause has become general, the high air pressure of the central area 

 is automatically reduced, and thus the blizzard gradually brings about its own 

 extinction. To the warming effect of the descending air current there is 

 rather suddenly added the latent heat of condensation of the moisture when 

 it is precipitated in the form of fine ice crystals within the air layers just 

 above the snow-ice surface. The rather sudden termination of the blizzard 

 may be thus in part explained. David has suggested that a ' hydraulic ram 

 efifect ' may be induced in the air of the upper currents, since the steam 

 clouds over Erebus, normally the antitrades, are temporarily reversed in 

 direction at the termination of a bUzzard, and for a short interval blow 

 northward." 



Source of the Precipitated Snoiv. — The actual initiation of the 

 strong wind may begin very suddenly, as has been especially empha- 

 sized by Simpson*^ and even more strikingly brought out by Maw- 

 son.** Referring to the source of the moisture of the blizzard as 

 the cirri, I stated in 191 1 : 



" There is, however, the probability that in general this snow or ice is 

 adiabatically melted and vaporized during its descent to the plateau, and 

 subsequently congealed as it mixes with the cold air above the plateau 



^- " Characteristics of Existing Glaciers," pp. 269-270. 



*3 " Scott's Last Expedition," Vol. 2, p. 325. 



44 Mawson, " The Home of the Blizzard," Vol. i. Chap. VII. 



