CHAPTER VI. 



PRESSURE, WINDS AND WEATHER. 



Until quite recently it was the general idea that from the belt of high pressure in about 

 latitude 35° S. pressure decreased right up to the South Polo. In other words that the 

 pressure distribution around the South Pole was very similar to that found in an ordinary 

 cyclone, the isobars running parallel to the circles of latitude and the centre of the system 

 at the Polo itself. According to this idea the ' roaring forties ' were simply the high westerly 

 winds which under such a distribution of pressure would circulate completely round the globe. 



Sailors however realised that the winds in the region of the ' roaring forties ' were any- 

 thing but consistently from the west. For example the Terra Nova on her journey from 

 Cape Town to Australia in 39° S. and 35° E. encountered a strong east-south-easterly gale 

 with wind forces up to 9 on the Beaufort Scale. In fact over the Southern Ocean the winds 

 back and veer with changes of the barometer exactly as they do in other temperate regions. 

 Further, Antarctic expeditions have shown that the pressure is lowest near to 60° S. latitude, 

 beyond which the barometer rises as far as observations have been made at sea-level towards 

 the Pole, and over the southern half of the ocean there is an easterly wuid as strong and 

 persistent as the westerly wind over the northern half. 



Thus the Gauss Expedition during her stay of eleven months in 66° S. had easterly 

 winds, often extremely strong, during 73 per cent, of the total time. Naturally when the 

 results of the expeditions which were in the Antarctic during 1902-04 came to be discussed 

 the old conceptions had to be revised. The most important work in this revision was done 

 by Hepworth, Lockyer and Meinardus, all of whom were writing at approximately the same time. 

 The conclusions of all three are practically the same in one important particular, namely, that 

 over the Southern Ocean there is a constant succession of true cyclonic depressions passing 

 from west to east having westerly winds on their northern sides and easterly on their southern 

 sides, giving rise respectively to the ' roaring forties ' north of latitude 60° S. and strong 

 easterly winds south of this latitude. 



The conclusions of Hepworth and Lockyer are in general agreement, but as Lockyer 

 has carried his investigations further than Hepworth we will consider his work in some detail* 



Lockyer first plots the mean daily barometer at a large number of stations, then assuming 

 that the rises and falls of the resulting curves correspond to the passage of high or low 

 pressure systems — anticyclones and cyclones — he adjusts curves of neighbouring stations imtil 

 the chief maxima and minima agree as far as possible. If curve A agrees best with curve 

 B when their time scales differ by say three days, then three days is considered to be the 

 average time the pressure systems take to travel from A to B. In this way he obtains 

 the average rate of travel of the pressure systems in each part of the southern hemisphere. 

 The next step is to find the intensity of the pressure changes at each place. This is done 

 by comparing the average difference between the successive maxima and minima on the 

 pressure curves. The method used is somewhat arbitrary but gives a satisfactory cjualitative 



* ' Southern Hemisphere Surface Air Circulation,' by J. S. Lockyer. Published by the Solar Physics Com- 

 mittee, 1910. 



