•240 



PRESSURE, WINDS AND WEATHER. 



Victoria Land consists of a range of mountains between 6,000 and 10,000 feet high which 

 runs from the south of Cape Adare in a north-westerly direction to Cape North- This 

 range limits the air motion to be parallel to itself. Also the promontory of high land on 

 the west side of which the station was situated would protect it from all winds between 

 south-east and north- Thus when a depression is approaching Cape Adare from the west the 

 wind cannot flow along the isobars from the north or north-east as would be the case away 

 from the land, but a flow of air passes from the north of the Ross Sea over the hills 

 into Robertson Bay when it becomes at Cape Adare a wind from the south-east. Now 

 when winds are constrained to travel directly from the high to the low pressure in this 

 way, they attain under a given gradient a much higher wind velocity than they would if they 

 were left free to travel along the isobars under the deflecting force due to the earth's rotation. 



The map for 8 a.m. on December 30th shows all these features clearly. The wind 

 at the ship is nearly parallel to the isobars and so from the N.N.E. The isobars at Cape 

 Adare are from the north-east to the south-west, but the wind is constrained to travel 

 parallel to the range of mountains and is therefore from the south-east. The air motion is 

 nearly directly across the isobars and is therefore much more violent than it would be if it 

 were not constrained to move in this direction, hence while the ship recorded only force 4, 

 force 8 was recorded at Cape Adare. 



When the depression has passed the isobars run from the south-east to the north-west, 

 the wind can then travel along them and as the storm moves away to the east the wind 

 velocity falls while the direction changes little. Thus while a depression is approaching the 

 station from the west the winds are very high, but as soon as it passes the wind velocity 

 decreases, but the wind remains from some direction near S.E. the whole time. We should 

 therefore expect the hurricanes to occur while the barometer is very low at Cape Adare 

 and to cease almost as soon as the pressure commences to rise ; the observations show that 

 this is the general rule. 



From this example we are justified in assuming that each dip in the pressure curve 

 at Cape Adare which is superposed on the main pressure waves and accompanied by high 

 southerly winds indicates the passage of a cyclone. To these we m^ay also add the few cases 

 in which a liigh mnd occurs at Cape Adare mthout any marked change of the barometer. 



The following is a list of all the occasions on which depressions of this nature have 

 left clear indications on the meteorological records taken at Cape Adare : — ■ 



Table 129. 

 Depressions passing Gape Adare. April-December 1911. 



