INTfllTf>OPICAL fHOhlT 



COLD monT 



SHEAHim FRONT 



Map 7. 



This gives rise to a seasonal front between 138° W., 35° S. and 155° W., 

 27° S., as is well shown by the map of precipitation effectiveness in 

 spring. The highest precipitation effectiveness known for this zone in 

 spring is found at Rapa (Map 5). 



In the summer the small central anticyclone weakens and moves 

 farther south (Map 3), so that no records are available except some signs 

 of higher nebulosity (U.S.W.B., 1938). In autumn the area of high pre- 

 cipitation effectiveness reappears with greater intensity, extending 

 farther to the north-west to include the Tubuai or Austral Islands as 

 well as Rapa (Map 4). It is especially in late autumn (May) that pre- 

 cipitation effectiveness is high and it is at this time of the year that winds 

 follow a complicated pattern (U.S.W.B., 1938) which has all the require- 

 ments for frontogenesis from Rapa to Tubuai. 



In winter (Map 7b) the small central anticyclone tends to disappear 

 between the two great anticyclones and Rapa finds itself just along the 

 line of frontogenesis (135° W.^, 35° S. to 147° W., 28° S.) while the nearby 

 islands have the usual incidence of fronts from travelling depressions 

 as are found in these latitudes in winter. However high the winter 

 precipitation effectiveness of Lord Howe Island, Norfolk Island, the 

 Kermadecs, and Pitcairn maj/^ be it is not as high as that of Rapa, where 

 actual frontogenesis takes place (Map 6). It is interesting to note that 

 a front in the vicinity of Rapa had been postulated by Bergeron (Miller, 

 1946) and by Haurwitz and Austin (1944). 



99 



